infra: add force build flag for host dependencies

infra: add force build flag for host dependencies

This commit adds a config option which will force buildroot to
build all host dependencies even if they are already present on the
host system. This may be a desirable option if different hosts are
used to build the same source. In this case, some packages will be
built on one host that are not built on another. This is problematic
if build source archives are cached afterwards for offline builds.

Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Bryce Ferguson <bryce.ferguson@rockwellcollins.com>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
[Peter: reword, drop exit 1, reshuffle]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>