linux: restrict output image formats to the appropriate arches

Some Linux kernel output image formats are available on some archs,
some not. For example 'uImage' is not supported on MIPS, so let's
prevent the user from making this selection.

Issue initially reported by Choi, David <David.Choi@Micrel.Com>.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>