procps-ng: Fix installation and take precedence over busybox

Currently procps-ng binaries are installed under $(TARGET_DIR)/usr/usr
instead of $(TARGET_DIR)/usr, which would be the expected. This is
caused because the default value of the --exec-prefix configure option
is /usr, and Buildroot always adds --prefix=/usr to the configure
option. We fix this by setting --exec-prefix to an empty value.

At the same time we make sure the procps-ng binaries have precedence
over the busybox ones.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Markos.Chandras <Markos.Chandras@imgtec.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>