uboot-imx

Makefile: move more flags to the top Makefile

Before this commit, most of compiler flags were defined in config.mk.
But it is redundant because config.mk is included from all recursive make.

This commit moves many complier flags to the top Makefile
and export them.
And we use new vaiarables to store them:
KBUILD_CPPFLAGS, KBUILD_CFLAGS, KBUILD_AFLAGS.
This will allow us to switch more smoothly to Kbuild.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>