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/cl-som-imx7_defconfig b/configs/cl-som-imx7_defconfig
index faad487..3de5284 100644
--- a/configs/cl-som-imx7_defconfig
+++ b/configs/cl-som-imx7_defconfig
@@ -22,6 +22,8 @@
 CONFIG_IMX_BOOTAUX=y
 CONFIG_SPI_BOOT=y
 CONFIG_BOOTDELAY=3
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="echo SD boot attempt ...; run sdbootscript; run sdboot; echo eMMC boot attempt ...; run emmcbootscript; run emmcboot; echo USB boot attempt ...; run usbbootscript; "
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x80