util/docker: Update docker files for coreboot-sdk & jenkins builders

util/docker: Update docker files for coreboot-sdk & jenkins builders

- Check out the specific toolchain version we want before building
the toolchain (This version uses 1.42).
- Add additional libraries and tools needed to build coreboot related
packages.
- Move everything required to build any of the coreboot or related
packages into the coreboot-sdk from coreboot-jenkins-node Dockerfile.
- Separate the text of the commands in the Dockerfiles.
- Use nproc to get the number of processors for building the toolchain
- Add some additional comments about why things are done the way that
they are to the README
- Update the version of coreboot-sdk that coreboot-jenkins-node uses to
1.42. (This matches the toolchain version)
- Move ccache setup from jenkins-node to coreboot-sdk.
- Update the maintainer.

Change-Id: I293285ef72e3e70259355d924d425fea98ee773d
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/16239
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>