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/am64x_evm_r5_defconfig b/configs/am64x_evm_r5_defconfig
index f943e1f..342e552 100644
--- a/configs/am64x_evm_r5_defconfig
+++ b/configs/am64x_evm_r5_defconfig
@@ -26,6 +26,8 @@
 CONFIG_SPL_SPI=y
 CONFIG_SPL_LOAD_FIT=y
 CONFIG_SPL_LOAD_FIT_ADDRESS=0x80080000
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="run distro_bootcmd"
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SPL_SIZE_LIMIT_SUBTRACT_GD=y
 CONFIG_SPL_SIZE_LIMIT_SUBTRACT_MALLOC=y