Revamp image to build dependencies as part of stages (#83)

Fixes #17.

Stage 2 is fairly minimal, stage 3 builds/installs OpenCV and WPILib et al, and stage 4
builds/installs the FRCVision webdash and adds the vision examples.

Other changes:
- OpenCV compiled with ffmpeg, OpenBLAS, and libgtk (fixes #79, fixes #80)
- OpenBLAS added to image (fixes #65)
- C++ Makefile is more easily extensible (fixes #71)
- Sources for everything are bundled into image into /usr/src
- README updated (fixes #16)
- pkg-config files for wpilibc et al are now installed and C++ Makefile uses them (if compiled local to Pi)
- Both dynamic and static libs are included in image

The only downside of all these changes (particularly the ffmpeg, OpenBLAS, and libgtk inclusion)
is the image size is now over 3GB (800MB compressed). The previous image didn't quite fit on a
2GB card however.

scripts/common

@@ -95,7 +95,7 @@ export -f on_chroot
 update_issue() {
 	local GIT_HASH
 	GIT_HASH=$(git rev-parse HEAD)
-	echo -e "Raspberry Pi reference ${IMG_DATE}\nGenerated using pi-gen, https://github.com/RPi-Distro/pi-gen, ${GIT_HASH}, ${1}" > "${ROOTFS_DIR}/etc/rpi-issue"
+	echo -e "FRCVision ${IMG_DATE}\nGenerated using pi-gen, https://github.com/allwpilib/FRCVision-pi-gen, ${GIT_HASH}, ${1}" > "${ROOTFS_DIR}/etc/rpi-issue"
 }
 export -f update_issue