Convert CONFIG_USE_BOOTCOMMAND et al to Kconfig

This converts the following to Kconfig:
   CONFIG_USE_BOOTCOMMAND
   CONFIG_BOOTCOMMAND
   CONFIG_RAMBOOTCOMMAND
   CONFIG_NFSBOOTCOMMAND

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/phycore-imx8mp_defconfig b/configs/phycore-imx8mp_defconfig
index 758c288..367f0d7 100644
--- a/configs/phycore-imx8mp_defconfig
+++ b/configs/phycore-imx8mp_defconfig
@@ -24,6 +24,8 @@
 CONFIG_SPL_LOAD_FIT=y
 # CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_OF_SYSTEM_SETUP=y
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="mmc dev ${mmcdev}; if mmc rescan; then if run loadimage; then run mmcboot; else run netboot; fi; fi;"
 CONFIG_DEFAULT_FDT_FILE="oftree"
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_SPL_BOARD_INIT=y