mkimage: Add map_sysmem() and IH_ARCH_DEFAULT to simplfy building

These are not actually used in mkimage itself, but the image code (which
is common with mkimage) does use them. To avoid #ifdefs in the image code
just for mkimage, define dummy version of these here. The compiler will
eliminate the dead code anyway.

A better way to handle this might be to split out more things from common.h
so that mkimage can include them. At present any file that mkimage uses
has to be very careful what headers it includes.

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