graph-depends: optimize execution speed

Until now, graph-depends was calling "make <pkg>-show-depends"
individually for eack package, which was very slow. Now, it calls
"make <pkg1>-show-depends <pkg2>-show-depends ... <pkgN>-show-depends"
for all packages it knows, and then does that recursively. It reduces
the number of make invocations to the deepest dependency chain in the
current configuration, instead of having a number of make invocations
equal to the number of enabled packages.

For a configuration with xvkbd enabled (which brings a significant
number of X.org dependencies) and a tar root filesystem, the time to
execute graph-depends was:

real	5m14.944s
user	4m53.590s
sys	0m14.069s

After our optimizations, it is now:

real	0m33.096s
user	0m30.878s
sys	0m1.472s

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>