bootm: Move fixup_silent_linux() earlier in the bootm stages

Before the bootm refactor, fixup_silent_linux() was done only in the
monolithic bootm case, not in the subcommand case. With the refactor, it
is done always, which is good. Unfortunately it is done too late, since it
is the PREP or CMDLINE stages that set up the command line for Linux.

Move fixup_silent_linux() into the LOADOS stage, which is where we find
out the OS being used, and can thus decide whether to perform this step.

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