cmake: toolchain: bintools abstraction

GNU/bintools abstraction completed.
The following commands and flags are now available in bintools:
- memusage
- elfconvert
- dissassembly
- readelf
- strip

The bintools template has now been updated with more description which
will be useful when adding support for more toolchains in future.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>