bootm: Update fixup_silent_linux() to return an error

At present this function fails silently on error. Update it to produce
an error code. Report this error to the user and abort the boot, since it
likely will prevent a successful start.

No tests are added at this stage, since additional refactoring is taking
place in subsequent patches.

Signed-off-by: Simon Glass <sjg@chromium.org>
3 files changed