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/mx6memcal_defconfig b/configs/mx6memcal_defconfig
index 6dd64b6..6da7a0e 100644
--- a/configs/mx6memcal_defconfig
+++ b/configs/mx6memcal_defconfig
@@ -14,6 +14,7 @@
 CONFIG_SPL=y
 CONFIG_SYS_MEMTEST_START=0x10000000
 CONFIG_SYS_MEMTEST_END=0x20000000
+# CONFIG_BOOTM is not set
 CONFIG_SUPPORT_RAW_INITRD=y
 CONFIG_SPL_SYS_MALLOC=y
 CONFIG_SPL_WATCHDOG=y
@@ -21,7 +22,6 @@
 CONFIG_SYS_MAXARGS=32
 CONFIG_SYS_PBSIZE=528
 # CONFIG_CMD_BOOTD is not set
-# CONFIG_CMD_BOOTM is not set
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_XIMG is not set