arm-v7m: call wfi when idling
arm-v7m: call wfi when idling

When there is nothing to execute, call wfi to save power. Disable wfi when
running with CONFIG_DEBUG in order to keep JTAG usable.

Signed-off-by: Fabien Parent <2a93cde040adee823a667eed2882d05f12a84573@gmail.com>

mtitinger/phabos,mtitinger/phabos