[POWERPC] Select DEFAULT_UIMAGE for HPC-NET/8641

I suspect this was meant to be added like it was to a whole slew of
other u-boot based boards, but probably just fell through the cracks.

Add "select DEFAULT_UIMAGE" for the 8641/HPC-NET.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>