Add an API+CLI to inject metadata for bootable OSTree commits

I was doing some rpm-ostree work and I wanted to compare two
OSTree commits to see if the kernel has changed.  I think
this should be a lot more natural.

Add `ostree commit --bootable` which calls into a new generic
library API `ostree_commit_metadata_for_bootable()` that
discovers the kernel version and injects it as an `ostree.linux`
metadata key.  And for extra clarity, add an `ostree.bootable`
key.

It's interesting because the "core" OSTree layer is all about
generic files, but this is adding special APIs around bootable
OSTree commits (as opposed to e.g. flatpak as well as
things like rpm-ostree's pkgcache refs).

Eventually, I'd like to ensure everyone is using this and
hard require this metadata key for the `ostree admin deploy`
flow - mainly to prevent accidents.

tests/admin-test.sh

@@ -65,6 +65,12 @@ assert_not_file_has_content status.txt "pending"
 assert_not_file_has_content status.txt "rollback"
 validate_bootloader
 
+# Test the bootable and linux keys
+${CMD_PREFIX} ostree --repo=sysroot/ostree/repo --print-metadata-key=ostree.linux show testos:testos/buildmaster/x86_64-runtime >out.txt
+assert_file_has_content_literal out.txt 3.6.0
+${CMD_PREFIX} ostree --repo=sysroot/ostree/repo --print-metadata-key=ostree.bootable show testos:testos/buildmaster/x86_64-runtime >out.txt
+assert_file_has_content_literal out.txt true
+
 echo "ok deploy command"
 
 ${CMD_PREFIX} ostree admin --print-current-dir > curdir

tests/basic-test.sh

@@ -21,7 +21,7 @@
 
 set -euo pipefail
 
-echo "1..$((86 + ${extra_basic_tests:-0}))"
+echo "1..$((87 + ${extra_basic_tests:-0}))"
 
 CHECKOUT_U_ARG=""
 CHECKOUT_H_ARGS="-H"
@@ -226,6 +226,13 @@ $OSTREE commit ${COMMIT_ARGS} -b test2-no-parent -s '' --parent=none $test_tmpdi
 assert_streq $($OSTREE log test2-no-parent |grep '^commit' | wc -l) "1"
 echo "ok commit no parent"
 
+cd ${test_tmpdir}
+if $OSTREE commit ${COMMIT_ARGS} -b test-bootable --bootable $test_tmpdir/checkout-test2-4 2>err.txt; then
+    fatal "committed non-bootable tree"
+fi
+assert_file_has_content err.txt "error: .*No such file or directory"
+echo "ok commit fails bootable if no kernel"
+
 cd ${test_tmpdir}
 # Do the --bind-ref=<the other test branch>, so we store both branches sorted
 # in metadata and thus the checksums remain the same.

tests/libtest.sh

@@ -415,11 +415,14 @@ setup_os_repository () {
     echo "an hmac file" > ${hmac_path}
     bootcsum=$(cat ${kernel_path} ${initramfs_path} | sha256sum | cut -f 1 -d ' ')
     export bootcsum
+    bootable_flag=""
     # Add the checksum for legacy dirs (/boot, /usr/lib/ostree-boot), but not
     # /usr/lib/modules.
     if [[ $bootdir != usr/lib/modules/* ]]; then
         mv ${kernel_path}{,-${bootcsum}}
         mv ${initramfs_path}{,-${bootcsum}}
+    else
+        bootable_flag="--bootable"
     fi
 
     echo "an executable" > usr/bin/sh
@@ -439,12 +442,12 @@ EOF
     mkdir -p usr/etc/testdirectory
     echo "a default daemon file" > usr/etc/testdirectory/test
 
-    ${CMD_PREFIX} ostree --repo=${test_tmpdir}/testos-repo commit --add-metadata-string version=1.0.9 -b testos/buildmaster/x86_64-runtime -s "Build"
+    ${CMD_PREFIX} ostree --repo=${test_tmpdir}/testos-repo commit ${bootable_flag} --add-metadata-string version=1.0.9 -b testos/buildmaster/x86_64-runtime -s "Build"
 
     # Ensure these commits have distinct second timestamps
     sleep 2
     echo "a new executable" > usr/bin/sh
-    ${CMD_PREFIX} ostree --repo=${test_tmpdir}/testos-repo commit --add-metadata-string version=1.0.10 -b testos/buildmaster/x86_64-runtime -s "Build"
+    ${CMD_PREFIX} ostree --repo=${test_tmpdir}/testos-repo commit ${bootable_flag} --add-metadata-string version=1.0.10 -b testos/buildmaster/x86_64-runtime -s "Build"
 
     cd ${test_tmpdir}
     rm -rf osdata-devel
@@ -453,7 +456,7 @@ EOF
     cd osdata-devel
     mkdir -p usr/include
     echo "a development header" > usr/include/foo.h
-    ${CMD_PREFIX} ostree --repo=${test_tmpdir}/testos-repo commit --add-metadata-string version=1.0.9 -b testos/buildmaster/x86_64-devel -s "Build"
+    ${CMD_PREFIX} ostree --repo=${test_tmpdir}/testos-repo commit ${bootable_flag} --add-metadata-string version=1.0.9 -b testos/buildmaster/x86_64-devel -s "Build"
 
     ${CMD_PREFIX} ostree --repo=${test_tmpdir}/testos-repo fsck -q