Currently it is not possible to override PREFIX when installing micropython
using the makefile. It is common practice to be able to run something like
this:
$ make install PREFIX=/usr DESTDIR=/tmp/staging
This fixes such usage.
Invoking "make" will still build the standard "micropython" executable, but
other variants are now build using, eg, "make VARIANT=minimal". This
follows how bare-metal ports specify a particular board, and allows running
any make target (eg clean, test) with any variant.
Convenience targets (eg "make coverage") are provided to retain the old
behaviour, at least for now.
See issue #3043.
