Added EC variant of active board 20210504

When building the active boards of type 20210504 for the ATLAS ITk strip end-cap powerboard testing, @dsperlic used other chips than nominally. Therefore some I2C addresses are different w.r.t. to the nominal version. This MR adds another active board type, called 20210504_EC which corrects the I2C assignments for this active board variant. Furthermore, the active_board_test I2C addresses to be tested are adapted.

NOTE! The AMAC IDs per carrier board slot were made unique! This must be ported to the other carrier board types, too, if wanted to be used with end-cap powerboards.

Tagging @kkrizka