Support calling *#6# to get IMEI

This makes it so if you "call" *#06# you get your IMEI in a nice blue box. I've verified that the IMEI displayed on my Pinephone is the correct one (the IMEI is also physically etched on to the CPU). If you have multiple SIM cards inserted, it displays all of your IMEIs. (although I'm unable to test that since my phone only has one SIM slot)

For reference, ETSI TS 122 030 section 6.7 says:

The following procedure shall instruct the ME to display its IMEI:

*#06#

The procedure shall be accepted and performed with and without an inserted SIM/USIM. The ME shall then display the 14 digits of the IMEI (not including the spare digit), the Check Digit and optionally the Software Version Number as defined in 3GPP TS 22.016 [4] (as a single string, in that order).

(AFAIK there are no phone OSes that display their Software Version Number when calling *#06#)