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/colibri_imx7_defconfig b/configs/colibri_imx7_defconfig
index 129f625..34c29ec 100644
--- a/configs/colibri_imx7_defconfig
+++ b/configs/colibri_imx7_defconfig
@@ -14,7 +14,7 @@
 CONFIG_IMX_HAB=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_BOOTDELAY=1
-# CONFIG_USE_BOOTCOMMAND is not set
+CONFIG_BOOTCOMMAND="run ubiboot ; echo ; echo ubiboot failed ; run distro_bootcmd;"
 CONFIG_USE_PREBOOT=y
 CONFIG_PREBOOT="setenv fdtfile ${soc}-colibri-${fdt_board}.dtb "
 # CONFIG_CONSOLE_MUX is not set