x86, boot: follow standard Kbuild style for compression suffix

When generating the compression suffix in
arch/x86/boot/compressed/Makefile, follow standard Kbuild
conventions, that is:

- Use a dash not underscore before y/m/n endings
- Use := whenever possible.

Requested-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>