Consider switching build directory

[microdev1]

Use build/board instead of build-board, i.e. move boards build to a sub-directory of build in the root of the port.

[dhalbert]

This seems fine to me. But I did a quick search for build- in the tree, and there are number of mentions, including possibly in submodules (adabot), for instance, so we'll need to change places that are assuming the old locations.

The top-level build directory (for documentation) is _build, which I kind of like: it'sis easy to spot. Maybe _build/<board>?

[jepler]

As you do this, please consider providing

• makefile variables that identify important targets e.g., $(FIRMWARE_ELF) $(FIRMWARE_BIN) $(FIRMWARE_UF2) $(FIRMWARE_MAP)
• phony rules that make them (e.g., firmware-elf: $(FIRMWARE_ELF) marked as .PHONY: firmware-elf

For instance right now I have my own additions that include things like

.PHONY: boot
boot: build-$(BOARD)/firmware.uf2
        _douf2 "$(MOUNT)" "$<" "$(SERIAL)"

where _douf2 is a script that does the serial trick, locates the bootloader drive, & copies the uf2 to it. I'll have to change this rule but being able to make the dependency boot: $(FIRMWARE_UF2) would be a simplification that would then be resilient to future changes.