boot,bootloader: add support for shim fallback and setting EFI boot variables on install

[valentindavid]

This is #13205 rebased onto #13412

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.53558% with 76 lines in your changes are missing coverage. Please review.

Project coverage is 78.84%. Comparing base (953fad1) to head (4d43110).

Files	Patch %	Lines
boot/assets.go	31.70%	25 Missing and 3 partials :warning:
boot/setefibootvars_linux.go	73.78%	20 Missing and 7 partials :warning:
gadget/mountedfilesystem.go	80.00%	10 Missing and 5 partials :warning:
bootloader/grub.go	80.64%	4 Missing and 2 partials :warning:

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Additional details and impacted files

@@            Coverage Diff             @@
##           master   #13511      +/-   ##
==========================================
- Coverage   78.85%   78.84%   -0.02%     
==========================================
  Files        1043     1044       +1     
  Lines      134592   134848     +256     
==========================================
+ Hits       106137   106321     +184     
- Misses      21839    21896      +57     
- Partials     6616     6631      +15     

Flag	Coverage Δ
unittests	78.84% <71.53%> (-0.02%)	:arrow_down:

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[ernestl]

@valentindavid @pedronis changed this to target 2.63 as agreed.

[valentindavid]

@olivercalder Could you have a look at the commits that are on top of your two commits and see if that makes sense to you.

The main one is https://github.com/snapcore/snapd/pull/13511/commits/7be3e78303fa4273cba1ee3037db16ea01f45a60

The rest is just some refactoring. Of the tests specially. And I can squash it later.

[ernestl]

Not ready for 2.63, moving to 2.64

[bboozzoo]

needs a rebase

[ernestl]

@alfonsosanchezbeato, please do a final pass