mgmt/mcumgr: Add CONFIG_IMG_MGMT_FRUGAL_LIST

The commit adds Kconfig options that enables "frugal status list";
when enabled, the status list will be sent with zero, empty and false
values omitted, slightly reducing number of bytes sent as response
from device.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>