Convert CONFIG_BOOTM_NETBSD to Kconfig

This converts the following to Kconfig:
   CONFIG_BOOTM_NETBSD

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/deneb_defconfig b/configs/deneb_defconfig
index 5377b4f..c0e7725 100644
--- a/configs/deneb_defconfig
+++ b/configs/deneb_defconfig
@@ -37,6 +37,7 @@
 CONFIG_AUTOBOOT_STOP_STR="\x1b\x1b"
 CONFIG_AUTOBOOT_KEYED_CTRLC=y
 CONFIG_CMD_CPU=y
+# CONFIG_BOOTM_NETBSD is not set
 # CONFIG_CMD_EXPORTENV is not set
 # CONFIG_CMD_IMPORTENV is not set
 # CONFIG_CMD_CRC32 is not set