Compile against the Broadcom's EGL/GLESv2 libs on the Raspberry Pi

Fix the GLESv2 and EGL locations in qmake's linux.conf and remove
pkg-config checks which would detect mesa's libgl. Don't use Qt's
raspberry pi config, because that would trigger its cross-compilation
config. Manually include the rpi hooks in the eglfs plugin.

This fixes the eglfs plugin on the Raspberry Pi.