circuitpython makefile should not default BOARD setting.

[mrmcwethy]

Between myself and sommersoft, we spent over an hour missing the most obvious typo. To start a metro M4 build, I typed

make BUILD=metro_m4_express

The build worked and generated a uf2 file. I moved it over into the metro M4 express board and it failed to connect via USB. We were blind to the parameter should have been BOARD not BUILD.

[tannewt]

Thanks @mrmcwethy !