kconfig: constify XPM data

Constify arrays as well as strings.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

kconfig: split images.c out of qconf.cc/gconf.c to fix gconf warnings

Currently, images.c is included by qconf.cc and gconf.c.
qconf.cc uses all of xpm_* arrays, but gconf.c only some of them.
Hence, lots of "... defined but not used" warnings are displayed
while compiling gconf.c

Splitting out images.c fixes the warnings.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>