Move mkdir build-dir to materials

There was only 1 page for creating the build dir as it used to be
creating a partition (which isn't needed for embedded) but it fits
better within materials as that's where ${CLFS}/cross-tools/sources gets
created anyway.  Let's keep it all together.
Retitle ch2 to include build dir creation
materials: Move sources dir create with ${CLFS}

So that ${CLFS} and ${CLFS}/sources are created together.
materials: remove arch specific packages & patches

The core build does not currently have any arch specific patches or
packages and shouldn't.

Core packages and patches are the same for each arch and should only
consist of:
* gcc
* binutils
* libc
* linux
* busybox or similar

Beyond that and it's into the beyond sections.
materials: Make everything common

Further make the materials section fully common.
Delete arm abi choice

Default is aapcs-linux and that's a good choice so leave it alone.
arm-fpu: Update recommendations, remove wrong fpus

Update recommendations to:
ARM9 with soft
Cortex-A with hard

Remove soft-vfp and none fpu types, they aren't valid.