imx: imx8mn-beacon: Move environment definition to env file

Instead of cluttering up a header file with a bunch of defines,
move the default environmental variables to a file called
imx8mn_beacon.env and reference it from the defconfigs.

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/imx8mn_beacon_fspi_defconfig b/configs/imx8mn_beacon_fspi_defconfig
index dade877..6c626ae 100644
--- a/configs/imx8mn_beacon_fspi_defconfig
+++ b/configs/imx8mn_beacon_fspi_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_GPIO=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_ENV_SOURCE_FILE="imx8mn_beacon"
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_SF_DEFAULT_SPEED=40000000
 CONFIG_ENV_SIZE=0x2000