sunxi: board: Use a more descriptive variable name

sunxi: board: Use a more descriptive variable name

The variable "cmp_str" always leaves me wondering if it is the DT name
of the current board (yes) or DT name in the FIT config entry (no).

In preparation for expanding the functionality here, rename it to
something that obviously means "this is the DT name we are looking for".

Acked-by: Maxime Ripard <[email protected]>
Reviewed-by: Andre Przywara <[email protected]>
Signed-off-by: Samuel Holland <[email protected]>
Reviewed-by: Jagan Teki <[email protected]>