when parallel build is enabled, both docker-fpm-frr and docker-syncd-brcm
is built at the same time, docker-fpm-frr requires swss which requires to
install libsaivs-dev. docker-syncd-brcm requires syncd package which requires
to install libsaibcm-dev.

since libsaivs-dev and libsaibcm-dev install the sai header in the same
location, these two packages cannot be installed at the same time. Therefore,
we need to serialize the build between these two packages. Simply uninstall
the conflict package is not enough to solve this issue. The correct solution
is to have one package wait for another package to be uninstalled.

For example, if syncd is built first, then it will install libsaibcm-dev.
Meanwhile, if the swss build job starts and tries to install libsaivs-dev,
it will first try to query if libsaibcm-dev is installed or not. if it is
installed, then it will wait until libsaibcm-dev is uninstalled. After syncd
job is finished, it will uninstall libsaibcm-dev and swss build job will be
unblocked.

To solve this issue, _UNINSTALLS is introduced to uninstall a package that
is no longer needed and to allow blocked job to continue.

Signed-off-by: Guohan Lu <lguohan@gmail.com>