boot/uboot: Set MAKE_ENV for kconfig build system

U-Boot fails to build in a GitLab CI context because the kconfig-package
build stage is unable to find bison or flex even though they are
installed in HOST_DIR.

To fix this, set UBOOT_MAKE_ENV so that UBOOT_KCONFIG_MAKE uses the
correct PATH.

Signed-off-by: Thomas Preston <thomas.preston@codethink.co.uk>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>