setting I2C pins leads to unworking hex file

[fabianhugo]

In the branch minidal setting the I2C pins according to the calliope mini pinout [1.X-2.X] results in a hex file that does not run the REPL terminal.
From line 179 of yotta_modules/mbed-classic/targets/hal/TARGET_NORDIC/TARGET_MCU_NRF51822/TARGET_NRF51_MICROBIT/PinNames.h the updated code would be:

    //SDA (SERIAL DATA LINE)
    I2C_SDA0 = p20,

    //SCL (SERIAL CLOCK LINE)
    I2C_SCL0 = p19

The hex file can be compiled, but the REPL console does not react.
The target reset pin of the microbit happens to be at p19 but also uncommenting the following line does not resolve the issue.

    //TARGET RESET
    TGT_NRESET = p19,