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]

(From OE-Core rev: a3d223284854a21c84e8f6d075d23b32789afa01)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/oe-buildenv-internal

@@ -114,7 +114,7 @@ BB_ENV_EXTRAWHITE_OE="MACHINE DISTRO TCMODE TCLIBC HTTP_PROXY http_proxy \
 HTTPS_PROXY https_proxy FTP_PROXY ftp_proxy FTPS_PROXY ftps_proxy ALL_PROXY \
 all_proxy NO_PROXY no_proxy SSH_AGENT_PID SSH_AUTH_SOCK BB_SRCREV_POLICY \
 SDKMACHINE BB_NUMBER_THREADS BB_NO_NETWORK PARALLEL_MAKE GIT_PROXY_COMMAND \
-SOCKS5_PASSWD SOCKS5_USER SCREENDIR STAMPS_DIR"
+SOCKS5_PASSWD SOCKS5_USER SCREENDIR STAMPS_DIR BBPATH_EXTRA"
 
 BB_ENV_EXTRAWHITE="$(echo $BB_ENV_EXTRAWHITE $BB_ENV_EXTRAWHITE_OE | tr ' ' '\n' | LC_ALL=C sort --unique | tr '\n' ' ')"