Make tempfs size configurable via new 'build.config' file

[fwilhe]

In some cases, the current hard-coded size limit for tempfs makes it impossible to build images with the builder. This change introduces a new config interface via a 'build.config' file which builder users might use to override selected variables.

Note that this introduces a breaking change in the sense that the build script needs to define the tempfs_size variable like in this example

# Default values which can be overriden via 'build.config' file
tempfs_size=2G

if [[ -f "$PWD"/build.config ]]; then
    . "$PWD"/build.config
fi

make_opts+=("TEMPFS_SIZE=$tempfs_size")

If this is not done, an error like Makefile:25: *** 'TEMPFS_SIZE undefined'. Stop. will occur.

Fixes #54

Needed for https://github.com/gardenlinux/dev-image/pull/3

[Vincinator]

Design decisions in this PR:

• build.config is a release artefact
• build.config is optional (for local builds)
• defaults are set in the builder build script

ToDos:

• The build.config is another interface that is used to control/configure the builder for a build. Ideally the build.config will be the only interface the developer needs to know. We should streamline this and adapt documentation.

Review: Looks good to me. Agree with the design decisions.