riotboot: define if building the bootloader

[basilfx]

Contribution description

This PR exposes a global define that can be used to skip certain parts of the bootloader, when building the bootloader.

I need that for several reasons:

• reducing the size of the bootloader (for instance, the EFM32 cpu_init() does a lot more that should not happen in the bootloader)
• skipping parts in cpu_init() that are not relevant during boot
  • configuring clock sources
    • example: I could have an updated firmware that uses another source/configuration
  • e.g. chip applying errata
    • example: there can be new (conflicting) EFM32 errata
  • configuring DC-DC parameters
    • example: I decide to update DC-DC parameters to make my board more efficient or stable
  • peripheral initialization

My goal is to boot as quick as possible to the actual firmware on the defaults, and then do proper initialization.

Testing procedure

Any test that uses the riotboot bootloader should still work.

Issues/PRs references

#11940, #8902 (comment)

[benpicco]

I'd say squash.

[basilfx]

@benpicco Squashed and rebased.

[miri64]

Something went wrong on murdock:

/bin/sh: 1: ./.murdock: not found

[benpicco]

The change is tiny yet very useful - ACK.
Only adds a new define, no side effects.

[basilfx]

Thanks!