u-boot.inc: Support menuconfig for u-boot

u-boot.inc: Support menuconfig for u-boot

u-boot mainline supports menuconfig, this patch enables menuconfig for
u-boot using cml1.

u-boot recipe in OE-core handles multi configuration of u-boot using
UBOOT_CONFIG. Since we cannot determine on which u-boot config the
menuconfig should run, the patch limits when UBOOT_CONFIG is not
defined.

(From OE-Core rev: 11278e3b2c75be80645b9841763a97dbb35daadc)

Signed-off-by: Manjukumar Matha <manjukumar.harthikote-matha@xilinx.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>