kbuild: clear VENDOR variable to fix build error on tcsh

Since the environment "VENDOR" is set in tcsh, it must be cleared in our
makefile.  Otherwise, boards without CONFIG_SYS_VENDOR fail to build:

  > make CROSS_COMPILE=arm-linux-gnueabi- wandboard_quad_defconfig all
    [ snip ]
    AR      arch/arm/lib/lib.a
    CC      arch/arm/lib/eabi_compat.o
  scripts/Makefile.build:55: /home/foo/u-boot/board/unknown/wandboard/ \
  Makefile: No such file or directory
  make[2]: *** No rule to make target `/home/foo/u-boot/board/unknown/ \
  wandboard/Makefile'.  Stop.
  make[1]: *** [board/unknown/wandboard] Error 2
  make: *** [__build_one_by_one] Error 2

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reported-by: Tom Everett <tom@khubla.com>
Reported-by: Jeroen Hofstee <jeroen@myspectrum.nl>