rpi-base.inc: Introduce DISABLE_VC4GRAPHICS

Since we enabled vc4graphics by default[1], there is no easy way to disable
it if needed. This patch introduces a variable (DISABLE_VC4GRAPHICS)
which when set to '1' will not add 'vc4graphics' to MACHINE_FEATURES
forcing the old behaviour.
One could use it in 'local.conf' for example.

[1] https://github.com/agherzan/meta-raspberrypi/pull/417

Signed-off-by: Andrei Gherzan <andrei@gherzan.ro>

conf/machine/include/rpi-base.inc

@@ -70,7 +70,7 @@ KERNEL_IMAGETYPE_DIRECT ??= "zImage"
 KERNEL_IMAGETYPE ?= "${@bb.utils.contains('RPI_USE_U_BOOT', '1', \
         '${KERNEL_IMAGETYPE_UBOOT}', '${KERNEL_IMAGETYPE_DIRECT}', d)}"
 
-MACHINE_FEATURES += "apm usbhost keyboard vfat ext2 screen touchscreen alsa bluetooth wifi sdio vc4graphics"
+MACHINE_FEATURES += "apm usbhost keyboard vfat ext2 screen touchscreen alsa bluetooth wifi sdio ${@bb.utils.contains('DISABLE_VC4GRAPHICS', '1', '', 'vc4graphics', d)}"
 
 # Raspberry Pi has no hardware clock
 MACHINE_FEATURES_BACKFILL_CONSIDERED = "rtc"