Makefile.include: allow overwriting flash-recipe

[cladmi]

Contribution description

This allows changing the flashing commands from the outside or in a BSP.

Implementation

It cannot be set as define flash-recipe ?= as the feature is only introduced only in gnumake 4.
Also this allows re-using default-flash-recipe if wanted.

Testing procedure

Flashing still works for a board that flashes. (CI will test this)

It can be overwritten:

make --no-print-directory -C examples/hello-world/ flash-only flash-recipe="echo changed recipe"
echo changed recipe
changed recipe

Issues/PRs references

Could be used for #11093 without introducing a one flasher feature for the moment.

I need to overwrite flash-recipe to handle flashing on another machine in my setup and currently need to rely on override for setting the variable.

This would allow refactoring the esp flashing command to use commands on different lines
https://github.com/RIOT-OS/RIOT/blob/master/cpu/esp32/Makefile.include

[cladmi]

Updated I was missing the $( ) ><

[cladmi]

@jcarrano if you want to have a look, I fixed the missing evaluation.

[jcarrano]

Squash and we go.

[jcarrano]

I removed "Disable cache" and "Run tests" since it has been already tested, and there's no point in wasting CI resources in a squash.

[cladmi]

Squashed.

[cladmi]

This one was pending and no changes have been done around that variable since then.