boot state: add ability to block state transitions

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>

src/include/bootstate.h

@@ -156,6 +156,14 @@ int boot_state_sched_on_entry(struct boot_state_callback *bscb,
 int boot_state_sched_on_exit(struct boot_state_callback *bscb,
                              boot_state_t state);
 
+/* Block/Unblock the (state, seq) pair from transitioning. Returns 0 on
+ * success < 0  when the phase of the (state,seq) has already ran. */
+int boot_state_block(boot_state_t state, boot_state_sequence_t seq);
+int boot_state_unblock(boot_state_t state, boot_state_sequence_t seq);
+/* Block/Unblock current state phase from transitioning. */
+void boot_state_current_block(void);
+void boot_state_current_unblock(void);
+
 /* Entry into the boot state machine. */
 void hardwaremain(int boot_complete);