Up to now, the semaphore was locked in core1_wait_ready() and later
freed in core1_load().  This is problematic when only core1_wait_ready()
is called (e.g. due to an invalid payload).

Instead, free the semaphore before leaving core1_wait_ready() and
reaquire it in core1_load().  This way an incorrect call sequence cannot
lead to a lockup.

Ref: #228