oe-buildenv-internal: add BBPATH_EXTRA to BB_ENV_EXTRAWHITE_OE
So that user can easily make their own conf files such as conf/site.conf
work. For example, poky's default BBPATH in bblayers.conf is:
BBPATH = "${TOPDIR}"
Will change it to:
BBPATH_EXTRA ??= ""
BBPATH = "${BBPATH_EXTRA}${TOPDIR}"
When user sets BBPATH_EXTRA in env to their own dir which contains
conf/site.conf, it will work.
Note, BBPATH_EXTRA must end with ":", we can't set BBPATH as
"${BBPATH_EXTRA}:${TOPDIR}" since the sanity would fail when
BBPATH_EXTRA is null.
[YOCTO #7837]
Signed-off-by: Robert Yang <liezhi.yang@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
