package_rpm: Add custom extension support for spec generation.

package_rpm: Add custom extension support for spec generation.

Add hooks to allow customisation of the rpm spec files. Since python functions
aren't visible in the data store, one variable is used to trigger the call to
a separately named function. A dummy function is not provided since this then
triggers various class ordering complexities which are best avoided.

Ultimately this will be replaced by a refactor of the code to generate the
spec file using a python class.

This allows the tizen layer to add hooks for the security manifests for
example.

(From OE-Core rev: 03ac91815013c0e85c4694b3ab849257e658aeba)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>