board: siemens: iot2050: Fix M.2 detection

The "simpler" the logic, the higher the probability to not test and get
things wrong, again: The absence of a "-PG2" suffix is not sufficient to
derive that we are on PG1. There is also "IOT2050-ADVANCED-M2".

Finally fix that by exactly matching against the two PG1 device names.

While changing this, we can also drop the not really needed check for
!board_is_sr1 in board_is_m2 and call the boards by their names
("board_is_pg1").

Reported-and-tested-by: Bao Cheng Su <baocheng.su@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>