including shell in the build breaks the code

[cooked]

when CONFIG_SHELL=y is used the samples won't start

[cooked]

possibly related to clash between the SHELL being on an uart... however not yet fully debugged.

temporary FIX:

• enable SHELL only if on SPI and disable shell on UART (to be done at Cmake / Kconfig level )
• in the samples for uart test use the nucleo onboard button instead shell commands (to be implemented in the src code)