Use -DSHIELD= to enable building of a shield

[erwango]

Is your enhancement proposal related to a problem? Please describe.
Current way of selecting a shield is to enable matching Kconfig symbol, cf

zephyr/samples/shields/x_nucleo_iks01a1/prj.conf

Line 2 in bd34448

CONFIG_SHIELD_X_NUCLEO_IKS01A1=y

This is no more possible if were going to run DT before Kconfig .

Describe the solution you'd like
We need an alternate way to select a shield.
The only option that I can think of right now is to implement a new -DSHIELD= build option to enable selection and build of a shield.

Describe alternatives you've considered
NA

Additional context
Follow up of #10821

[erwango]

@SebastianBoe, would you have time to check this ?

[SebastianBoe]

I agree with this approach.

[SebastianBoe]

When the set of CMake variables and their inter-related dependencies grows in size we might want to consider creating a Kconfig tree specifically for DT configuration.

But for now CMake variables will have to do.

[erwango]

@galak , I'm working on #12092 which is more tricky than expected.
Will probably not be able to address this point before next year