common: Move bootm_decomp_image() to image.c (as image_decomp())

Upcoming patches want to add decompression to use cases that are no
longer directly related to booting. It makes sense to retain a single
decompression routine, but it should no longer be in bootm.c (which is
not compiled for all configurations). This patch moves
bootm_decomp_image() to image.c and renames it to image_decomp() in
preparation of those upcoming patches.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
[trini: Fix warning around handle_decomp_error being unused]
Signed-off-by: Tom Rini <trini@konsulko.com>