Fix NuttX build under Cygwin

[MaEtUgR]

Describe problem solved by this pull request
NuttX build using the Cygwin Toolchain is broken since the NuttX 9.1.0+ upgrade #15139 . The pr was merged nevertheless to unblock core developers using the new NuttX version and with the intent to fix the build shortly after. PX4 stable v1.11 was the officially recommended version to build in case you ran into any problems.

Fixes #15760

Describe your solution
It took me quite some time to figure out:

• NuttX has a new way of configuring the build environment now using multiple flags
• Those flags have to be set before including Config.mk
• The new way of assembling ARCHINCLUDES even before the makefile made a new way of converting the include paths that are already inside compile flags necessary
• Backslashes in the compile flags for some reason get parsed and have to be escaped. I did not invest time in findign exactly where that happens but it's most likely within the NuttX build system and when escaping them it all works fine.

Test data / coverage
So far I build px4_fmu-v4 locally but didn't do any hardware tests yet.

Additional context

[MaEtUgR]

@davids5 I shifted these additions up because they need to get converted as well and added spaces between -I and the path for simpler makefile parsing.

[davids5]

@MaEtUgR - Thank you for fixing this!

[davids5]

@MaEtUgR - can we re-enable the windows CI build now?

[MaEtUgR]

@davids5 Yes, @dagar was so nice and turned it on again for prs 👍

[lukegluke]

@MaEtUgR thanks for work, windows builds ok again.
I notice that now (after moving to latest PX4 with upgraded NuttX) it takes more time to build, even if there is nothing to build. Previously there was such output: "nothing to build", now it always prints [1/4] Generating apps/libapps.a, apps/platform/.built at least.

[MaEtUgR]

@lukegluke Thanks for the feedback. I'm currently assuming that's a general build system issue and not specific to the Windows toolchain. We should check that. Could you make a separate issue for that problem and tag me?

[lukegluke]

Done!