ostree-prepare-root: Fix running with musl

musl libc's implementation of `realpath` works by opening the path and then
doing a lookup in `/proc/self/fd` to find the canonical path.  This fails
if `/proc` is not mounted.  This causes problems for us if
`ostree-prepare-root` is `init` as `/proc` won't be mounted.

We have to mount `/proc` anyway for `/proc/cmdline` so this fix just
expands the scope over which `/proc` is mounted to include both our
`realpath` calls.

See also:

* http://www.openwall.com/lists/musl/2016/06/08/2 and
* http://git.musl-libc.org/cgit/musl/tree/src/misc/realpath.c?id=e738b8cbe64b6dd3ed9f47b6d4cd7eb2c422b38d

Closes: #485
Approved by: cgwalters

tests/test-switchroot.sh

@@ -4,7 +4,14 @@ this_script="${BASH_SOURCE:-$(readlink -f "$0")}"
 
 setup_bootfs() {
 	mkdir -p "$1/proc" "$1/bin"
-	echo "quiet ostree=/ostree/boot.0 ro" >"$1/proc/cmdline"
+
+	# We need the real /proc mounted here so musl's realpath will work, but we
+	# want to be able to override /proc/cmdline, so bind mount.
+	mount -t proc proc "$1/proc"
+
+	echo "quiet ostree=/ostree/boot.0 ro" >"$1/override_cmdline"
+	mount --bind "$1/override_cmdline" "$1/proc/cmdline"
+
 	touch "$1/this_is_bootfs"
 	cp "$(dirname "$this_script")/../ostree-prepare-root" "$1/bin"
 }