scripts: twister: Name prefix loading

Sped up Twister via name prefix loading.
If the only thing influencing the platform selection
is the platform filter (-p), then we only load *.yaml
files that start the same way as those selected platforms.
We split the platform name to the first '/' or '@'.

Signed-off-by: Lukasz Mrugala <lukaszx.mrugala@intel.com>