[POWERPC] bootwrapper: Decompress less, check more

Our kernels put everything in the first load segment, and we read that.
Instead of decompressing to the end of the gzip stream or supplied image
and hoping we get it all, decompress the expected size and complain if
it is not available.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>