cmake: Toolchain abstraction: Abstraction of binary tool, strip.

This abstracts the interface for generation of the strip command
line, by naming the desired actions instead of directly setting the
command parameters, which then opens up for other binary tool sets
which may require different arguments to achieve the desired result.

The intent here is to abstract Zephyr's dependence on toolchains,
thus allowing for easier porting to other, perhaps commercial,
toolchains and/or usecases.

No functional change expected.

Signed-off-by: Danny Oerndrup <daor@demant.com>