u-boot: Fix boot on rpi3
U-boot configured with 'rpi_2_config' does not boot on a RaspberryPi 3.
Instead we need to configure with 'rpi_3_32b_config'.
@@ -7,5 +7,6 @@ include conf/machine/raspberrypi2.conf
MACHINE_EXTRA_RRECOMMENDS += "linux-firmware-bcm43430"
+UBOOT_MACHINE = "rpi_3_32b_config"
SERIAL_CONSOLE = "115200 ttyS0"
VC4_CMA_SIZE ?= "cma-256"
