[jenkins] - fix comparison with empty strings
[jenkins] - obey the TARBALLS env variable for android builds
[jenkins] - use i686 toolchain for 32bit linux builds by default
[jenkins] - add a sane Default "configuration" for linux builds (Debug like for other platforms too)
[jenkins] - only explicitly set the android sdk version if its not set to "Default" (in which case configure uses the prefered version)