Allow using path for build_profile relative to custom.py

[NormPlum]

When compiling Godot (e.g. for generating a custom export template), you can specify the path to a build_profile (a file generated from Godot's Engine Compilation Configuration Editor). On the command line, this is done as follows:

scons target=template_release build_profile=path/to/profile.gdbuild

This path is relative to the directory that you're running scons from.

You can also put all your command line build options into a Python file and reference it like so:

scons target=template_release profile=path/to/custom.py

When you put the build_profile option into that file, it would be nice to use a path that's relative to the Python file, and not the scons command:

optimize = "size"
# Assuming profile.gdbuild is in the same directory as custom.py
build_profile = "profile.gdbuild"
production = "yes"