sched: minor cleanup

In sched_new.c, move the static functions to the top, so they do not
require forward declarations.  Additionally, fit all lines in 80
columns.

In sched_end.c, shorten long line.  It is allowed to remain above 80
columns because it contains a string.

Change-Id: I7536caebbeb4a8dceb940cd5e55aa0ee7da397ed
sched: new_task() takes period in microseconds

Instead of taking task period as platform and configuration specific
ticks in new_task(), take the period in microseconds.  new_task()
converts this value to ticks, which are still used by the remainder of
the scheduler.

new_task() will round up to the nearest number of ticks that can be
achieved by the scheduler.  For example, if the tick period is 250us,
and a task with a period of 200us is created, it will actually have a
period of 250us.

Change-Id: Ie985eb74b01e42454fc73deb78ddafea2c7a46c4