SCons will omit the file in parent dir of SConstruct somehow and build
the object files in that dir instead of in variant dir. This cause
problem because we cannot mix the object files between M0 and M4 which
SCons failed to rebuild. So we have to manually remove the files before
building.

Currently only lpc43xx is supported.

Some compiler is brain-damaged that it will yeild a warning for headers
not ended with a newline. Yes, I mean you, Keil.

M0 core is always booted by the M4 core. It means that if we are running
in M0, the clock is always configured.