BeagleBoardPkg: remove outdated build scripts and instructions

BeagleBoardPkg: remove outdated build scripts and instructions

Now that we can build the Beagleboard NOR image without the custom
build scripts, let's remove them since they are outdated and out of
sync with each other.

Remove readme.txt as well: it is also outdated, and mostly covers
QEMU and how to build Linaro SD images, which may not be relevant to
most users.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18211 6f19259b-4bc3-4df7-8a09-765794883524