boards: silabs: Consistently name starter kits

Rename all Silicon Labs starter kits to always use the official kit name
as board target name. Previous kit names used various naming schemes,
including putting part of the SoC name in the board name. With HWMv2,
SoC names (if needed) should go in the SoC board qualifier.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>