buildroot

gnupg: add host package

For E.G. post-build / post-image scripts as it generally cannot be expected
to be available on the build host and/or there are some differences in
behaviour between gnupg 1.x / 2.x.

Provide gnupg 1.x instead of 2.x, as it is simpler to build (less
dependencies) and easier to use in post-build / post-image scripts (E.G. no
gpg-agent that keeps running in the background).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>