configs/buildroot: set FORCE_UNSAFE_CONFIG

Fixes #33 (closed) by setting FORCE_UNSAFE_CONFIG to bypass a check against building as root in the host-tar package, to work around a build failure when using the Docker runtime on macOS.

FORCE_UNSAFE_CONFIGURE only affects the host-tar package. If set when running as root, then the following changes occur:

• REPLACE_MKNOD = 1
• MKNOD_FIFO_BUG = 1
• mknod() will be replaced with a wrapper (see …/buildroot/build/host-tar-1.35/gnu/mknod.c) that uses mkfifo() instead of mknod().

Seems like this is a harmless change as it's only affecting the host version of tar. It also looks like the replacement mknod() is functionally equivalent on Linux.