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: NSam Ravnborg <sam@ravnborg.org>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>

arch/x86/boot/compressed/Makefile

@@ -46,10 +46,10 @@ $(obj)/vmlinux.bin.bz2: $(vmlinux.bin.all-y) FORCE
 $(obj)/vmlinux.bin.lzma: $(vmlinux.bin.all-y) FORCE
 	$(call if_changed,lzma)
 
-suffix_$(CONFIG_KERNEL_GZIP)  = gz
-suffix_$(CONFIG_KERNEL_BZIP2) = bz2
-suffix_$(CONFIG_KERNEL_LZMA)  = lzma
+suffix-$(CONFIG_KERNEL_GZIP)	:= gz
+suffix-$(CONFIG_KERNEL_BZIP2)	:= bz2
+suffix-$(CONFIG_KERNEL_LZMA)	:= lzma
 
 LDFLAGS_piggy.o := -r --format binary --oformat $(CONFIG_OUTPUT_FORMAT) -T
-$(obj)/piggy.o: $(obj)/vmlinux.scr $(obj)/vmlinux.bin.$(suffix_y) FORCE
+$(obj)/piggy.o: $(obj)/vmlinux.scr $(obj)/vmlinux.bin.$(suffix-y) FORCE
 	$(call if_changed,ld)