In order to properly sequence the boot state machine it's
important that outside code can block the transition from
one state to the next. When timers are not involved there's
no reason for any of the existing code to block a state
transition. However, if there is a timer callback that needs to
complete by a certain point in the boot sequence it is necessary
to place a block for the given state.

To that end, 4 new functions are added to provide the API for
blocking a state.
1. boot_state_block(boot_state_t state, boot_state_sequence_t seq);
2. boot_state_unblock(boot_state_t state, boot_state_sequence_t seq);
3. boot_state_current_block(void);
4. boot_state_current_unblock(void);

Change-Id: Ieb37050ff652fd85a6b1e0e2f81a1a2807bab8e0
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/3204


Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>