A recent change broke the 'bootm' command on sandbox. The root cause is
using a pointer as an address. Conversion from pointer to address needs to
use map_to_sysmem() so that sandbox can do the right thing. The problem was
pre-existing but uncovered by a recent commit.

Fix this. Also move fit_get_end() to the C file to avoid needing to include
mapmem.h (and thus asm/io.h) everywhere.

Fixes: 1fec3c5d (common/image.c: Make boot_get_ramdisk() perform a check for Android images)

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