llext-edk: fix BOARD_TARGET variable generation to be Zephyr-compatible

The board revision is not part of the NORMALIZED_BOARD_TARGET variable
as composed by Zephyr, so it must also not be used in the EDK exported
value to avoid mismatches. The revision is exported as a separate
variable, so it can still be used to differentiate between board
revisions.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>