mk: introduce new install syntax

The old install command was:
	make install T=x86_64-native-linuxapp-gcc DESTDIR=install
It still works and can be replaced by these more standard commands:
	make config T=x86_64-native-linuxapp-gcc 0=x86_64-native-linuxapp-gcc
	make O=x86_64-native-linuxapp-gcc
	make install O=x86_64-native-linuxapp-gcc prefix= DESTDIR=install

It means the "make install" do not perform any compilation anymore when T
is not used. It is done only in pre_install to keep compatibility with the
old syntax based on T= option.

The default prefix /usr/local is empty in the T= case which is
used only for a local install.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Panu Matilainen <pmatilai@redhat.com>