bootm: Add a Kconfig option for bootm functionality

Create a separate Kconfig option which enables the bootm logic,
separate from the 'bootm' command. This will eventually allow booting
without CMDLINE enabled.

Update boards which disable CMD_BOOTM to disable BOOTM instead, since
CMD_BOOTM now depends on BOOTM

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/iot_devkit_defconfig b/configs/iot_devkit_defconfig
index 55ced6b..a09f3ed 100644
--- a/configs/iot_devkit_defconfig
+++ b/configs/iot_devkit_defconfig
@@ -14,12 +14,12 @@
 CONFIG_SYS_CLK_FREQ=16000000
 CONFIG_SYS_LOAD_ADDR=0x30000000
 CONFIG_LOCALVERSION="-iotdk-1.0"
+# CONFIG_BOOTM is not set
 # CONFIG_ARCH_FIXUP_FDT_MEMORY is not set
 CONFIG_SYS_PROMPT="IoTDK# "
 CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=280
 # CONFIG_CMD_BOOTD is not set
-# CONFIG_CMD_BOOTM is not set
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_XIMG is not set
 # CONFIG_CMD_LOADB is not set