parisc: Reintroduce option to gzip-compress the kernel

By adding the feature to build the kernel as self-extracting
executeable, the possibility to simply compress the kernel with gzip was
lost.

This patch now reintroduces this possibilty again and leaves it up to
the user to decide how the kernel should be built.

The palo bootloader is able to natively load both formats.

Signed-off-by: Helge Deller <deller@gmx.de>