scripts/install-buildtools: improvements

* Install directory defaults to scripts/../buildtools
  e.g. --directory is set by default
  This avoids the user having to type in their sudo password
  to install in /opt/poky/<installer-version>

* Use "." rather than "source" for sourcing the environment script
  as not all distros (e.g. Debian) have "source" by default.

* Add buildtools/ to .gitignore

* Fix typos in example usage (--install-version -> --installer-version)

[YOCTO #13832]

Signed-off-by: Tim Orling <timothy.t.orling@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>