sh: Make initrd detection more robust.

Various boot loaders go to various extents to thwart the initrd detection
logic (mostly on account of not being able to be bothered with adhering
to the established boot ABI), so we make the detection logic a bit more
robust. This makes it possible to work around the SDK7786's firmware's
attempts to thwart compressed image booting. Victory is mine.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>