[POWERPC] Add documentation for the zImage's gunzip convenience functions

This patch adds documenting comments to the gunzip convenience
functions added in commit ad9d2716

.
It also removes a stray newline, and an unused global variable.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>