devmand: skip unconfigured script invocations

	. if a up/down script isn't specified, devmand
	  would try to execute a command line with (null)
	  in it, causing messy messages on startup
	. skip script at a higher level when missing,
	  and add asserts for expected strings at the lower level

Change-Id: Ia0d772076f3781caa5879ea4e64b53fa6c6e8478
tps65950: allow 0 as id for qemu beaglexm

makes booting on qemu beaglexm a lot prettier.

Change-Id: I45d0383abbf0d21227a95f538ed78ca6c1ed7871