oe-buildenv-internal: Support extra settings

oe-buildenv-internal: Support extra settings

This can make it possible to do extra settings such as PATH for other layers,
for exampole, other layers may also have scripts directories as oe-core, we
have to run them with path since they are not in PATH, for example, there is a
tool meta-browser/scripts/firefox-gen-l10n-recipes, we have to use
/path/to/meta-browser/scripts to run it. Make oe-buildenv-internal check and
enable $OEROOT/extrasettings, and set a line like the following can fix the
problem:

PATH=/relative/to/$OEROOT/meta-browser/scripts:$PATH

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>