microkernel: use _thread_essential_set()

This wrapper exists so that the ESSENTIAL bit doesn't have to be set
explicitly in _nanokernel.current. (And a note is added to nano_init to
avoid a nasty gotcha there.)

Change-Id: I4026ac0cfeace60e36abdbccf15554759f2dbf7a
Signed-off-by: Geoff Thorpe <geoff.thorpe@nxp.com>