weston-launch requires linux-pam

The weston package should select the BR2_PACKAGE_LINUX_PAM package since linux-pam
is required when building weston-launch.

Aslo clean up the Weston configuration flags a bit.
WebKitGTK+ should look for rpi-userland EGL and GLES2 headers

WebKitGTK+ doesn't use pkg-config to locate EGL and GLESv2 headers but just
runs compilation tests to see if they're located in CFLAGS or CXXFLAGS.
Buildroot package should adjust WebKitGTK+ rules to that, utilizing pkg-config
to establish the rpi-userland staging installation that includes the necessary
headers.
WebKitGTK+ shouldn't build the WebKit1 layer

The focus is on WebKit2.
Add rpi_webkitgtk_defconfig

The rpi_webkitgtk_defconfig make target is added so it's easy to set up the
buildroot configuration to build WebKitGTK+ for Wayland.
Merge pull request #11 from zdobersek/webkitgtk_defconfig

WebKitGTK+ defconfig