unix/variants/standard: Fix role of PREFIX when used to install.

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.

ports/unix/variants/standard/mpconfigvariant.mk

@@ -4,8 +4,8 @@ PROG ?= micropython
 
 # install micropython in /usr/local/bin
 TARGET = micropython
-PREFIX = $(DESTDIR)/usr/local
-BINDIR = $(PREFIX)/bin
+PREFIX = /usr/local
+BINDIR = $(DESTDIR)$(PREFIX)/bin
 
 install: micropython
 	install -d $(BINDIR)