emgw3.conf: Fix u-boot config and use append style for MACHINEOVERRIDES

With the switch to kirkstone, the upstream BSP changed the u-boot config
used by the machine we include in emgw3.conf
Also let's use an append style for MACHINEOVERRIDES which makes it clearer

Signed-off-by: Florin Sarbu <florin@balena.io>

layers/meta-balena-fsl-arm/conf/machine/emgw3.conf

@@ -3,7 +3,7 @@
 #@SOC: i.MX8 MM
 #@DESCRIPTION: Machine configuration for Boundary EM GW 3
 
-MACHINEOVERRIDES = "nitrogen8mm:${MACHINE}"
+MACHINEOVERRIDES =. "nitrogen8mm:"
 
 include conf/machine/nitrogen8mm.conf
 
@@ -14,4 +14,5 @@ KERNEL_DEVICETREE = " \
 
 # Override default Nitrogen8MM u-boot configuration
 UBOOT_DTB_NAME = "imx8mm-em.dtb"
-UBOOT_CONFIG[sd] = "em_2gr0_defconfig"
+UBOOT_CONFIG = "emgw3"
+UBOOT_CONFIG[emgw3] = "em_2gr0_defconfig,sdcard"