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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index c24986c..75e6589 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -15,7 +15,7 @@
 CONFIG_LTO=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_BOOTDELAY=10
-# CONFIG_USE_BOOTCOMMAND is not set
+CONFIG_BOOTCOMMAND="mmc dev ${mmcdev}; if mmc rescan; then echo SD/MMC found on device $mmcdev; if run loadbootenv; then run importbootenv; fi; echo Checking if uenvcmd is set ...; if test -n $uenvcmd; then echo Running uenvcmd ...; run uenvcmd; fi; echo Running default loadimage ...; setenv bootfile zImage; if run loadimage; then run loadfdt; run mmcboot; fi; else run nandboot; fi"
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_SEPARATE_BSS=y
 # CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR is not set