grub: really disable splash screen support

When BR2_TARGET_GRUB_SPLASH is enabled, we were passing
--enable-graphics, but when it was disabled, we were not doing
anything. However, in Grub, graphics support is enabled by default,
and you have to pass --disable-graphics to disable it.

Fix this by passing --enable-graphics or --disable-graphics as
appropriate.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

boot/grub/grub.mk

@@ -22,7 +22,8 @@ GRUB_CFLAGS = \
 	-O0 -Wl,--build-id=none \
 	-fno-stack-protector
 
-GRUB_CONFIG-$(BR2_TARGET_GRUB_SPLASH) += --enable-graphics
+GRUB_CONFIG-y += $(if $(BR2_TARGET_GRUB_SPLASH),--enable-graphics,--disable-graphics)
+
 GRUB_CONFIG-$(BR2_TARGET_GRUB_DISKLESS) += --enable-diskless
 GRUB_CONFIG-$(BR2_TARGET_GRUB_3c595) += --enable-3c595
 GRUB_CONFIG-$(BR2_TARGET_GRUB_3c90x) += --enable-3c90x