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/nitrogen6q_defconfig b/configs/nitrogen6q_defconfig
index fca3e5f..9793c73 100644
--- a/configs/nitrogen6q_defconfig
+++ b/configs/nitrogen6q_defconfig
@@ -22,6 +22,8 @@
 CONFIG_AHCI=y
 CONFIG_SUPPORT_RAW_INITRD=y
 CONFIG_BOOTDELAY=3
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="run distro_bootcmd"
 CONFIG_USE_PREBOOT=y
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
 CONFIG_BOARD_EARLY_INIT_F=y