build: Move __NuttX__ definition to tools/Config.mk

[xiaoxiang781216]

Summary

ensure this critical macro get defined in all projects

Impact

No functional change

Testing

All build pass

[yamt]

lgtm

[patacongo]

For other systems things like this are defined by the tool chain.  Indeed  __NuttX__ is predefined by the  NuttX build root tool chain.Sent from Samsung tablet.

[xiaoxiang781216]

Yes, but most developer use the official toolchain, not NuttX build root toolchain. We have to consider this usecase too. A common location is needed here, otherwise the duplication will spread out all board's Make.defs or the compiler will fail in some external library which explicitly check __NuttX__. And the change isn't harmful byself because it's compatible with both toolchain.

[jerpelea]

LGTM

[masayuki2009]

@xiaoxiang781216
I noticed that this PR affected build time.

For example, spresense:wifi_smp with a parallel build

Before:
real	0m23.740s
user	1m16.792s
sys	0m18.236s

After:
real	0m28.574s
user	1m23.348s
sys	0m19.613s