switchroot: Be explicit about what could cause /sysroot to be ro

If you don't have rw in the kernel cmdline or have ro in it, often you
hit this issue. This is just to be really explicit about that in the
error messages so people can check.

Signed-off-by: Eric Curtin <ecurtin@redhat.com>

src/libotcore/otcore.h

@@ -60,6 +60,10 @@ GKeyFile *otcore_load_config (int rootfs, const char *filename, GError **error);
 // The directory holding the root overlayfs
 #define OSTREE_DEPLOYMENT_ROOT_TRANSIENT_DIR "root-transient"
 
+#define OTCORE_SYSROOT_NOT_WRITEABLE \
+  "sysroot.readonly=true requires %s to be writable at this point, the cmdline should contain rw " \
+  "but not ro, if that is not the case this is likely the issue"
+
 // The name of the composefs metadata root
 #define OSTREE_COMPOSEFS_NAME ".ostree.cfs"
 // The temporary directory used for the EROFS mount; it's in the .private directory

src/switchroot/ostree-prepare-root-static.c

@@ -229,8 +229,7 @@ main (int argc, char *argv[])
   if (sysroot_readonly)
     {
       if (!sysroot_currently_writable)
-        errx (EXIT_FAILURE, "sysroot.readonly=true requires %s to be writable at this point",
-              root_arg);
+        errx (EXIT_FAILURE, OTCORE_SYSROOT_NOT_WRITEABLE, root_arg);
       /* Pass on the fact that we discovered a readonly sysroot to ostree-remount.service */
       int fd = open (_OSTREE_SYSROOT_READONLY_STAMP, O_WRONLY | O_CREAT | O_CLOEXEC, 0644);
       if (fd < 0)

src/switchroot/ostree-prepare-root.c

@@ -557,8 +557,7 @@ main (int argc, char *argv[])
   if (sysroot_readonly)
     {
       if (!sysroot_currently_writable)
-        errx (EXIT_FAILURE, "sysroot.readonly=true requires %s to be writable at this point",
-              root_arg);
+        errx (EXIT_FAILURE, OTCORE_SYSROOT_NOT_WRITEABLE, root_arg);
     }
   /* Pass on the state for use by ostree-prepare-root */
   g_variant_builder_add (&metadata_builder, "{sv}", OTCORE_RUN_BOOTED_KEY_SYSROOT_RO,