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>