bitbake.conf: add definition for BUILDSDK_CXXFLAGS

${BUILDSDK_CXXFLAGS} was introduced since
commit 55c83cb239df5faf5e2143fffca47f2f16931cb3
cross.bbclass: override TARGET_* flags

bitbake.conf has definitions for both ${BUILDSDK_CPPFLAGS} and
${BUILDSDK_CFLAGS} but there is none for ${BUILDSDK_CXXFLAGS}.
This was a regression as in the past, CXXFLAGS is the same as
CFLAGS in SDK environment.

Adding definition for ${BUILDSDK_CXXFLAGS} will resolve CXXFLAGS
being set wrongly in SDK environment.

[YOCTO #11769]

(From OE-Core rev: 073ca1cbd1b70d5b36618584a8c88cbba391a259)

Signed-off-by: Chang Rebecca Swee Fun <rebecca.swee.fun.chang@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/conf/bitbake.conf

@@ -546,6 +546,7 @@ export CFLAGS = "${TARGET_CFLAGS}"
 export TARGET_CFLAGS = "${TARGET_CPPFLAGS} ${SELECTED_OPTIMIZATION}"
 
 export BUILD_CXXFLAGS = "${BUILD_CFLAGS}"
+BUILDSDK_CXXFLAGS = "${BUILDSDK_CFLAGS}"
 export CXXFLAGS = "${TARGET_CXXFLAGS}"
 export TARGET_CXXFLAGS = "${TARGET_CFLAGS}"