The patch introduces the CRITICAL feature of POST tests. If the test

marked as POST_CRITICAL fails then the alternative, post_critical,
boot-command is used. If this command is not defined then U-Boot
enters into interactive mode.

Signed-off-by: Dmitry Rakhchev <rda@emcraft.com>
Signed-off-by: Yuri Tikhonov <yur@emcraft.com>