stm32mp1: add runtime information in environment

Set board name with the first dts compatible found in DT
code under CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG

The result with DEVICE_TREE=stm32mp157c-ev1 is:
    STM32MP> env print
    	board=stm32mp1
    	board_name=stm32mp157c-ev1

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 7645470..33628b8 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1403,6 +1403,7 @@
 	select SYSRESET
 	select SYS_THUMB_BUILD
 	imply CMD_DM
+	imply ENV_VARS_UBOOT_RUNTIME_CONFIG
 	help
 	  Support for STM32MP SoC family developed by STMicroelectronics,
 	  MPUs based on ARM cortex A core