[findtoolchain] Update toolchain scanner for Yocto 3.0
[findtoolchain] Update toolchain scanner for Yocto 3.0

- the folder to find the toolchain in Yocto context has changed
  swi-mdm9x28-wp => swi_mdm9x28_wp-poky-linux

- the architecture of virt-x86 platform is now i686

Resolves: LXSWIREF-1649
Change-Id: I3c334a16f3cc8479eb6b362364570808e4200349

legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af