Raspberry PI build: Add a flag to exclude some of the plugins

For things like Raspberry PI we need to remove some of the more
memory intensive plugins or ones that require OpenGL (pending
Qt5).  To compile with this reduced set use this command:

make GCS_QMAKE_OPTS="CONFIG+=LIGHTWEIGHT_GCS" -j7 gcs
Rasbperry PI build: Add scope to the list of excluded builds
Raspberry PI: Update the lightweight set of plugins

Changes from Kipkool for building on the Raspberry PI.  See
https://github.com/PhoenixPilot/PhoenixPilot/pull/176
Merge pull request #176 from PhoenixPilot/james/raspberry_pi2

Raspberry PI build