unix/Makefile: Allow to install all variants of the executable.

The install target is current broken when PROG is used to override the
default executable name.  This fixes it by removing the redundant TARGET
variable and uses PROG directly instead.

The install and uninstall targets are also moved to the common unix
Makefile so that all variants can be installed in the same way.

ports/unix/Makefile

@@ -248,3 +248,12 @@ axtls: $(TOP)/lib/axtls/README
 $(TOP)/lib/axtls/README:
 	@echo "You cloned without --recursive, fetching submodules for you."
 	(cd $(TOP); git submodule update --init --recursive)
+
+PREFIX = /usr/local
+BINDIR = $(DESTDIR)$(PREFIX)/bin
+
+install: $(PROG)
+	install -D $(PROG) $(BINDIR)/$(PROG)
+
+uninstall:
+	-rm $(BINDIR)/$(PROG)

ports/unix/variants/standard/mpconfigvariant.mk

@@ -1,16 +1,3 @@
 # This is the default variant when you `make` the Unix port.
 
 PROG ?= micropython
-
-# install micropython in /usr/local/bin
-TARGET = micropython
-PREFIX = /usr/local
-BINDIR = $(DESTDIR)$(PREFIX)/bin
-
-install: micropython
-	install -d $(BINDIR)
-	install $(TARGET) $(BINDIR)/$(TARGET)
-
-# uninstall micropython
-uninstall:
-	-rm $(BINDIR)/$(TARGET)