bootm: vxworks: Support Linux compatible standard DTB for ARM and PPC

Enhance do_bootm_vxworks() to support Linux compatible standard DTB
for ARM and PPC, when the least significant bit of flags in VxWorks
bootargs is set. Otherwise it falls back to the existing bootm flow
which is now legacy.

Signed-off-by: Lihua Zhao <lihua.zhao@windriver.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>