populate_sdk_qt5: use _append operator
populate_sdk_qt5: use _append operator

populate_sdk assigns TOOLCHAIN_HOST_TASK and TOOLCHAIN_TARGET_TASK with ?=.
Thus depending on the order in which populate_sdk and populate_sdk-qt5 get
parsed the assignments from populate_sdk are discarded and the resulting sdk
is missing among others the environment-setup script.

Signed-off-by: Max Krummenacher <2adf29ceaf73d2cea8fff1ff57e7b25e74cc8f1f@toradex.com>
Signed-off-by: Martin Jansa <516df3ff67e1fa5d153800b15fb204c0a1a389dc@gmail.com>

sapiippo/meta-qt5,timopulkkinen/meta-qt5,meta-qt5/meta-qt5,meta-qt5/meta-qt5,sapiippo/meta-qt5,schnitzeltony/meta-qt5,timopulkkinen/meta-qt5,timopulkkinen/meta-qt5,kraj/meta-qt5,schnitzeltony/meta-qt5,timopulkkinen/meta-qt5,kraj/meta-qt5