Kconfig: Migrate CONFIG_BOOTM_* options

Migrate all of the existing OS support options that are under
CONFIG_BOOTM_* to Kconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/colibri-imx8qxp_defconfig b/configs/colibri-imx8qxp_defconfig
index a223a16..b809905 100644
--- a/configs/colibri-imx8qxp_defconfig
+++ b/configs/colibri-imx8qxp_defconfig
@@ -12,6 +12,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_CMD_CPU=y
+# CONFIG_BOOTM_NETBSD is not set
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_CLK=y