Add buildall script for testing many/most packages separately with build logs and documentation for this and mkpkg

The biggest update is to document the Makefile.autotools.in way of
writing .mk files.

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

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

Add support and documentation for out-of-tree builds with syntax
like the Linux kernel (make O=<dir>).

'default n' is the default, so there's no need to say it explicitly.

  TODO: document HAVE_MANPAGES and HAVE_INFOPAGES (and adjust line-numbers in the text accordingly)

- Add some more explanation to the -clean and -dirclean targets