build: add DESTDIR support

This change implements support for specifying an alternative installation
destination directory by setting DESTDIR when calling 'make install'.

Example: make DESTDIR=/alternative-rootfs install

It is particularly useful when cross-compiling and installing in a dedicated
directory where for example the rootfs or a build sandbox for the target
architecture is mounted.

By using DESTDIR you can avoid to mess with PREFIX to include the alternative
installation directory!