doc: add documentation of DESTDIR usage in INSTALL and make help

Document the newly introduced DESTDIR option.

Signed-off-by: Christian Wiese <chris@opensde.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
doc: fix make help for CROSS_COMPILE option

One doesn't need to specify a path for the CROSS_COMPILE option but
a prefix identifying the target architecture.

Signed-off-by: Christian Wiese <chris@opensde.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>