[OE-core] Usage of "elf" IMAGE_TYPES

[Saul Wold]

Folks,

I know it's late in our 2.4 cycle, but we recently discovered that
mkelfImage fails due to a recent upgrade to binutils. In doing some
research, mkelfimage was removed back in Oct 2014 from the coreboot
repo. It was replaced with cbfstool.

Unless I hear otherwise, I recommend that we mark elf Images as
deprecated for 2.4 and remove them in 2.5.

I have not yet dived into exactly what binutil change triggered the
failure.

Further thoughts?

Sau!