mk: fix verbose pmdinfogen run

mk: fix verbose pmdinfogen run

When building with "make V=1" it is expected to see the output of each
compiler command in order to debug them.
Unfortunately the pmdinfogen related commands were always quiet.

It is fixed by defining the commands in some Makefile variables.
They are printed if the verbose mode is enabled.

The other benefit of this rework is to stop compilation after a
failure with pmdinfogen.

The command readlink is removed in this rework because it seems useless.

Fixes: 3d781ca32874 ("mk: do post processing on objects that register a driver")

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>