batctl: pass the feature switches directly

batctl: pass the feature switches directly

The CONFIG_* switches were assembled into a newline separated string
and smuggled to the build through a shell variable via shexport, so
the Makefile read:

    MAKE_FLAGS += $$$$$(call shvar,batctl_config)

They can simply be listed on the make command line instead. The
result is identical, since later assignments on the command line
override earlier ones, exactly as the "n first, then y" order of the
old string relied on. The longest variant expands to 96 assignments,
about 2 KB, so there is no length concern that would justify the
indirection.

Signed-off-by: Josef Schlehofer <redacted>
Co-authored-by: Claude Fable 5 <redacted>