build: Allow fetching into directory outside build directory

[kubanrob]

Introduce an additional path (F, FETCH_BASE) that is used for fetch targets for the builds.

There are multiple motivations for using a distinct fetch base directory:

• Reusing downloads shared by multiple builds (eg. musl).
• Allow building without internet access (with populated fetch base).

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Ran the checkpatch.uk on your commit series before opening this PR;
• Updated relevant documentation.

Base target

• Architecture(s): N/A
• Platform(s): N/A
• Application(s): N/A

Additional configuration

Build the application with the desired target directory, for example:

make F=$(pwd)/../fetch-base

For testing purposes, fetching with wget can be disabled as following:

# instead of wget, call false, which always fails
make F=$(pwd)/../fetch-base WGET=false

Description of changes

Before, the buildsystem fetched archives into $BUILD_DIR/$LIBNAME. With this commit, archives are fetched into $FETCH_BASE/$LIBNAME. By default, $FETCH_BASE is set to $BUILD_DIR, resulting in the same behaviour as before.

Further details:

• Checksums are checked same as before.
• Incorrect checksums lead to the build failing, no retry.
• $FETCH_BASE does not have an effect on clone rules.