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/nokia_rx51_defconfig b/configs/nokia_rx51_defconfig
index 87f8ab9..fd1ae62 100644
--- a/configs/nokia_rx51_defconfig
+++ b/configs/nokia_rx51_defconfig
@@ -20,6 +20,8 @@
 CONFIG_BOOTDELAY=30
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_MENU_SHOW=y
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="run sdboot;run emmcboot;run attachboot;echo"
 CONFIG_USE_PREBOOT=y
 CONFIG_PREBOOT="run preboot"
 CONFIG_CONSOLE_MUX=y