README: Reword the section about github forking

This splits the lines in 80 cols and rework the text a little bit to
easy reading.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
nativesdk-packagegroup-qt5-toolchain-host: Add packagegroup

Allow easy inclusion of host packages for the Qt5 standalone SDK or
external toolchain.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
packagegroup-qt5-toolchain-target: Add packagegroup

Allow easy inclusion of target packages for the Qt5 SDK or external
toolchain.

The list of the target package is based on the Arago Project, which
has been contribute by Denys Dmytriyenko <denys@ti.com>.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
populate_sdk_qt5.bbclass: Allow generation of toolchain

This allow the toolchain generation using '-c populate_sdk' as it
includes the need host libraries into the SDK.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
meta-toolchain-qt5: add Qt5 SDK standalone meta recipe

Signed-off-by: Denys Dmytriyenko <denys@ti.com>