[build] add support for EFR32 devices on Thread 1.3

[lmnotran]

This adds support for building efr32 images for Thread 1.3

It also refactors the make-firmware.bash and make-reference-release.bash scripts to allow other platforms to be easily added.

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Main changes

• Add ot-efr32 to build script
• Add ot-efr32 submodule
• Skip Silabs apps in ot-efr32
• Limit efr32 to only brd4166a.
• Refactor ./script/make-firmware.bash
  • If no platform is specified, the script will build all platforms. If a platform is specified as an argument, the script will build only that platform

Other changes

• Allow relative paths for OUTPUT_ROOT when running make-firmware.bash directly
• Add support for downloading nrfutil on MacOS
• Use wildcard to simplify .gitignore

[lmnotran]

@jwhui - Could you add @romacdon and @suveshpratapa as reviewers?

[lmnotran]

CC: @edmont

[romacdon]

@jwhui can you answer @lmnotran's question above? Also, we'd appreciate it if you or someone from your team could review the changes as well. Thanks

[jwhui]

@jwhui can you answer @lmnotran's question above? Also, we'd appreciate it if you or someone from your team could review the changes as well. Thanks

@romacdon , you should be able to assign reviewers now. I have also assigned a couple of additional reviewers. Thanks.

[lmnotran]

Thanks for reviewing @superwhd @simonlingoogle !

@jwhui - I just addressed the nits. Should be good to go!