Makefile: be sure the default rule 'all:' is the first one

In the coming patch, we are going to change the order in which our rules
are defined, because we include the gendoc infra before we define the
'all:' rule, so we need to decalre the 'all:' rule before we include
gendoc.

Declare it very, very early in the Makefile, so it always kick in first.
The actual dependency is still declared much later, all that counts is
that "all:" is first.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>