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/mx51evk_defconfig b/configs/mx51evk_defconfig
index 06c43b7..7ee220a 100644
--- a/configs/mx51evk_defconfig
+++ b/configs/mx51evk_defconfig
@@ -10,6 +10,8 @@
 CONFIG_DEFAULT_DEVICE_TREE="imx51-babbage"
 # CONFIG_CMD_BMODE is not set
 CONFIG_SYS_LOAD_ADDR=0x92000000
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="mmc dev ${mmcdev}; if mmc rescan; then if run loadbootscript; then run bootscript; else if run loadimage; then run mmcboot; else run netboot; fi; fi; else run netboot; fi"
 CONFIG_USE_PREBOOT=y
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y