boot/uboot: Fix kconfig to use $(BR2_MAKE)

U-Boot must use $(BR2_MAKE) as it uses a Make feature from v4.0. We
already use $(BR2_MAKE) in the BUILD_CMDS, but the kconfig commands
still uses $(MAKE). Without this fix, building U-Boot with kconfig will
fail with the following cryptic error.

> Makefile:37: *** missing separator.  Stop.

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
1 file changed