Add support for explicitly requesting any specific bootloader type

...with the `sysroot.bootloader` configuration option.  This can be useful
when converting a system to use `ostree` which doesn't currently have a
bootloader configuration that `ostree` can automatically detect, and is
also useful in combination with the `--sysroot` option when provisioning a
rootfs for systems other than the one you're running `ostree admin deploy`
on.

src/libostree/ostree-repo-private.h

@@ -114,6 +114,9 @@ typedef enum {
 typedef enum {
   CFG_SYSROOT_BOOTLOADER_OPT_AUTO = 0,
   CFG_SYSROOT_BOOTLOADER_OPT_NONE,
+  CFG_SYSROOT_BOOTLOADER_OPT_GRUB2,
+  CFG_SYSROOT_BOOTLOADER_OPT_SYSLINUX,
+  CFG_SYSROOT_BOOTLOADER_OPT_UBOOT,
   CFG_SYSROOT_BOOTLOADER_OPT_ZIPL,
   /* Non-exhaustive */
 } OstreeCfgSysrootBootloaderOpt;
@@ -122,6 +125,9 @@ static const char* const CFG_SYSROOT_BOOTLOADER_OPTS_STR[] = {
   /* This must be kept in the same order as the enum */
   "auto",
   "none",
+  "grub2",
+  "syslinux",
+  "uboot",
   "zipl",
   NULL,
 };