Specify PICO_BOARD (and trailing whitespace).

[SeanBurford]

Specify PICO_BOARD in the build instructions.

If PICO_BOARD is not specified then dump_console and dump_rom are not built, since the default ("pico") does not have PICO_WYW43_SUPPORTED.

Many people will have PICO_BOARD set already, but it's worth documenting since the default behavior is to not build anything useful:

$ env -i PATH="$PATH" PICO_SDK_PATH="${PICO_SDK_PATH}" NEXMON_ROOT="${NEXMON_ROOT}" cmake ..

Using PICO_SDK_PATH from environment ('/home/user/src/pico-w/pico-sdk')
PICO_SDK_PATH is /home/user/src/pico-w/pico-sdk
Defaulting platform (PICO_PLATFORM) to 'rp2040' since not specified.
Defaulting target board (PICO_BOARD) to 'pico' since not specified.
Using board configuration from /home/user/src/pico-w/pico-sdk/src/boards/include/boards/pico.h
Pico Platform (PICO_PLATFORM) is 'rp2040'.

then:

$ make

[ 20%] Building ASM object pico-sdk/src/rp2040/boot_stage2/CMakeFiles/bs2_default.dir/compile_time_choice.S.o
[ 40%] Linking ASM executable bs2_default.elf
[ 40%] Built target bs2_default
[ 60%] Generating bs2_default.bin
[ 80%] Generating bs2_default_padded_checksummed.S
[100%] Building ASM object pico-sdk/src/rp2040/boot_stage2/CMakeFiles/bs2_default_library.dir/bs2_default_padded_checksummed.S.o
[100%] Built target bs2_default_library