userland: Add '--no-as-needed' linker flag

This is done to ensure all specified shared libraries are linked against
applications (e.g. raspivid)i.

libmmal_vc_client.so makes use of __attribute__(constructor) to ensure
that supplier components (e.g. camera) are loaded when the static library is
loaded.

raspivid, and possibly other applications, link against libmmal_vc_client.so,
causing the ctor to execute, but there is no needed dependency.

Some build environments (e.g. Yocto/OpenEmbedded) pass the '--no-as-needed'
linker flag which removes the dependency on libmmal_vc_client and thus
components are not registered.

In this situation raspivid then gives an error of the form
  root@raspberrypi:~# raspivid -o test
  mmal: mmal_component_create_core: could not find component 'vc.ril.camera'
  mmal: Failed to create camera component
  mmal: main: Failed to create camera component
  mmal: Failed to run camera app. Please check for firmware updates

For further details see:
https://lists.yoctoproject.org/pipermail/yocto/2014-June/019933.html

Change-Id: I9d787fd83379e8b6428dde3827395c8b96be1f7f
Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
Acked-by: Andrei Gherzan <andrei@gherzan.ro>
userland: Add SRCBRANCH and SRCFORK for use by SRC_URI

Makes it clearer to manage repo forks, and branches prior to (e.g.) upstream
commits of in tree changes.

Change-Id: I42f8af975c08f84c4ceb9910713eba2da3a87d8b
Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
Acked-by: Andrei Gherzan <andrei@gherzan.ro>