[PATCH] powerpc: Add CONFIG_DEFAULT_UIMAGE for embedded boards

Embedded boards that u-boot require a kernel image in the uImage format.
This allows a given board to specify it wants a uImage built by default.

This also fixes a warning at config time, as this symbol is referred
to in arch/powerpc/platforms/83xx/Kconfig.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>