pkg-generic.mk: strip leading/trailing spaces

The manual gives this example for using the github macro:

  FOO_VERSION = v1.0 # tag or full commit ID

Unfortunately, people copy/pasting this example will face weird make errors,
because it leads the FOO_VERSION variable to end with a space.  Similar
problems can happen when testing a version bump or similar, so strip
leading/trailing spaces before the version is used to construct the build
directory path.

Reported-by: Edd Robbins <edd.robbins@gmail.com>
Cc: Edd Robbins <edd.robbins@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>