cpu/efm32: move LOW_POWER_ENABLED to efm32-features.mk

[basilfx]

Contribution description

This PR moves LOW_POWER_ENABLED to a feature in efm32-features.mk. This 'centralizes' all (low-level) tunables at one place, and now allows one to simply add it to a Makefile.

I renamed this to EFM32_LEUART_ENABLED so it is clear that it's about the EFM32 CPU.

Testing procedure

The test in tests/cpu_efm32_features will ensure that it compiles.

Issues/PRs references

n/a

[benpicco]

I guess it makes sense to have this easily configurable through the Makefile.

No functional changes.

[benpicco]

Murdock is complaining that _is_usart() is now unused in tests/cpu_efm32_features.

Guard that function with the same #if EFM32_LEUART_ENABLED && defined(LEUART_COUNT) && LEUART_COUNT > 0 and you should be good to go.

Actually, this is pretty long, so better use something like

#if EFM32_LEUART_ENABLED && defined(LEUART_COUNT) && LEUART_COUNT > 0
#define USE_LEUART
#endif
…
#ifdef USE_LEUART
…

still room for improvement ;)

[benpicco]

Looks much better now, please squash! :)

Murdock found a trailing whitespace, remove it while you're squashing.

[basilfx]

Squashed, and fixed the trailing whitespace.

[benpicco]

Same reason as before.
No functional changes, just moves config option to Make variable.

[basilfx]

Thanks!