boards: silabs: Consistently name radio boards

Rename all Silicon Labs radio boards to always use the official kit name of
the kit containing only the radio board as board target name. Disentangle
boards using Series 1 and Series 2 SoCs that have almost nothing in common
in terms of devicetree content, but still occupied the same directory.

As part of the initial HWMv2 port, all radio boards were merged into a
single target efr32_radio, using the SoC as a board qualifier to target
specific boards. This was unfortunate, as the SoC name is not sufficient
to identify a specific board -- the same SoC may be used on multiple
different boards. This commit reinstates every radio board as an individual
board target.

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