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/imx8qm_mek_defconfig b/configs/imx8qm_mek_defconfig
index bf330ab..a432035 100644
--- a/configs/imx8qm_mek_defconfig
+++ b/configs/imx8qm_mek_defconfig
@@ -22,6 +22,8 @@
 CONFIG_SYS_LOAD_ADDR=0x80280000
 CONFIG_OF_SYSTEM_SETUP=y
 CONFIG_BOOTDELAY=3
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="mmc dev ${mmcdev}; if mmc rescan; then if run loadbootscript; then run bootscript; else if test ${sec_boot} = yes; then if run loadcntr; then run mmcboot; else run netboot; fi; else if run loadimage; then run mmcboot; else run netboot; fi; fi; fi; else booti ${loadaddr} - ${fdt_addr}; fi"
 CONFIG_LOG=y
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL_BOARD_INIT=y