cmake: Fix EXTRA_LDFLAGS feature

It is supported to add give extra flags to the linker from the
commandline like this:

cmake -DEXTRA_LDFLAGS=-Lmy_dir path

But unfortunately this was broken during the CMake
migration. Interestingly, the reason that it was broken is that KBuild
was also partially broken. KBuild would pass on EXTRA_LDFLAGS when
object files were linked together into built-in.o files, but it would
not use EXTRA_LDFLAGS for the final link into an elf file.

This patch fixes EXTRA_LDFLAGS.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>