POST: move CONFIG_POST to Makefiles

Introduce the new logical option CONFIG_HAS_POST which is set when the
platform has CONFIG_POST set. Use CONFIG_HAS_POST in the post/ Makefiles
to determine should the POST libs be compiled for the selected target
platform, or not.

To avoid breaking u-boot linking process, the empty post/libpost.a file is
created for platforms which do not have POSTs.

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>