imx: imx8mm-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
imx8mm_beacon.env and reference it from the defconfig.

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/imx8mm_beacon_defconfig b/configs/imx8mm_beacon_defconfig
index bb02b9b..52edb2e 100644
--- a/configs/imx8mm_beacon_defconfig
+++ b/configs/imx8mm_beacon_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_GPIO=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_ENV_SOURCE_FILE="imx8mm_beacon"
 CONFIG_SF_DEFAULT_SPEED=10000000
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0xFFFFDE00