build-sysroots: target or native sysroot population need to be selected explicitly

build-sysroots: target or native sysroot population need to be selected explicitly

Running them in parallel is prone to races as postinsts from target sysroots
rely on executables from native sysroots which may or may not be fully prepared
yet. This was observed for example here:

https://autobuilder.yoctoproject.org/typhoon/#/builders/146/builds/468/steps/12/logs/stdio
https://autobuilder.yoctoproject.org/typhoon/#/builders/147/builds/467/steps/12/logs/stdio

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>