Makefile: introduce KSTRIPCMD to strip kernel modules

When sstrip is selected it tries to strip kernel modules too.
Unfortunately this fails with a "unrecognized program segment header
size" error thus interrupting the build process.

We introduce a new $(KSTRIPCMD) strip command for this, being a regular
strip when sstrip is selected and an empty stub when not stripping.

At the same time get rid of the REMOVE_SECTION_* variables, as they are
only used once.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>