unicore32: simplify linker script generation for decompressor

When I was searching for unneeded $(KCONFIG_CONFIG) usages, I noticed
this strange build dependency.

It can use $(call if_changed,...) in case ZTEXTADDR and ZBSSADDR are
changed, but even a simpler way is to use the pattern rule in
scripts/Makefile.build. This is what arch/arm/boot/compressed/Makefile
does.

I did only build test. I confirmed equivalent vmlinux.lds was generated.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>