buildroot

documentation: update about new package infrastructures

Document the new generic package infrastructure, and how target/host
packages are handled, both for the generic package infrastructure and
the autotools package infrastructure.

This documentation replaces the documentation that used to be present
at the top of Makefile.autotools.in. Both tutorial and reference
documentation are provided.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>