common/cmd_bootm.c: seperate do_bootm_vxworks related code from CONFIG_CMD_ELF.

do_bootm_vxworks now is available under the configuration option
CONFIG_BOOTM_VXWORKS, thus aligned with other operating systems
that supported by bootm command. The bootvx command still depneds
on CONFIG_CMD_ELF.

Signed-off-by: Miao Yan <miao.yan@windriver.com>