boot: introduce MCUBOOT_CPU_IDLE() for support low power single thread

Introduced MCUBOOT_CPU_IDLE() macro.
If a port supports single thread or is bare-metal then might be need
to switch to idle mode explicitly form MCUboot code.
The call allows to enable lower power consumption while waiting for
incoming transmission in serial recovery etc.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>