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/imx7_cm_defconfig b/configs/imx7_cm_defconfig
index 3f8f618..a1aa680 100644
--- a/configs/imx7_cm_defconfig
+++ b/configs/imx7_cm_defconfig
@@ -21,7 +21,7 @@
 CONFIG_IMX_RDC=y
 CONFIG_IMX_BOOTAUX=y
 CONFIG_DISTRO_DEFAULTS=y
-CONFIG_BOOTCOMMAND="run findfdt; run finduuid; run distro_bootcmd"
+CONFIG_BOOTCOMMAND="run boot${boot-mode}"
 CONFIG_DEFAULT_FDT_FILE="ask"
 # CONFIG_BOARD_EARLY_INIT_F is not set
 CONFIG_SPL_I2C=y