Add property `build_platform`
Add property `build_platform`

To make easier for us to remove the hardcoded entries that we have in our build system

rnixx/python-for-android,PKRoma/python-for-android,PKRoma/python-for-android,rnixx/python-for-android,rnixx/python-for-android,kronenpj/python-for-android,kivy/python-for-android,rnixx/python-for-android,kronenpj/python-for-android,rnixx/python-for-android,rnixx/python-for-android,PKRoma/python-for-android,kivy/python-for-android,PKRoma/python-for-android,kivy/python-for-android,germn/python-for-android,germn/python-for-android,kronenpj/python-for-android,germn/python-for-android,germn/python-for-android,kronenpj/python-for-android,germn/python-for-android,germn/python-for-android,kivy/python-for-android,PKRoma/python-for-android,kronenpj/python-for-android,kivy/python-for-android