Add SDK target to build an easy-to-use SDK

Run
$> bitbake meta-rpi-sdk
to get a SDK for building Mozilla without the usage of OpenEmbedded.
b2g-linuxgl: Add some necessary dependencies
b2g-linuxgl: Adjust mozconfig to newest Gecko
rpi-b2g-image: Install wifi tools
Add development image

The development image contains the same tools as the "production"
image, but additional development tools and debug symbols and library
headers for all installed libraries.