uboot-phytec

Makefile: drop unused cpp_cfg macro

Commit e19b0fb4851f (kbuild: generate u-boot.cfg as a byproduct of
include/autoconf.mk) removed the use of the cpp_cfg macro in Makefile,
but forgot to remove its definition.

Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>