buildroot

network-manager: don't test if the binary exists in the init script

The test doesn't make sense. It just exits without any error if the
binary doesn't exist, which is silly.

Replace the NETWORKMANAGER_BIN variable, which was used only once, by
the full path of the binary file.

Drop the now useless variables prefix, exec_prefix and sbindir.

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>