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/mx28evk_auart_console_defconfig b/configs/mx28evk_auart_console_defconfig
index d57db5f..22310f5 100644
--- a/configs/mx28evk_auart_console_defconfig
+++ b/configs/mx28evk_auart_console_defconfig
@@ -14,6 +14,8 @@
 CONFIG_SPL=y
 CONFIG_SYS_LOAD_ADDR=0x42000000
 CONFIG_SYS_EXTRA_OPTIONS="MXS_AUART,MXS_AUART_BASE=MXS_UARTAPP3_BASE"
+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_SYS_CONSOLE_IS_IN_ENV=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_ARCH_MISC_INIT=y