kconfig: move CONFIG_CMD_BOOTM to Kconfig

CONFIG_CMD_BOOTM is defined in config_cmd_defaults.h
which is forcebly included from each board.
So, the default value of "config CMD_BOOTM" should be "y".

For some boards undefining it (bf506f-ezkit, controlcenterd_TRAILBLA,
controlcenterd_TRAILBLAZER_DEVELOP, controlcenterd_TRAILBLAZER),
"# CONFIG_CMD_BOOTM is not set" should be added to their defconfig.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>