mk: fix link with glibc < 2.17

mk: fix link with glibc < 2.17

There is a dependency on librt with old glibc.
The -lrt option was added everywhere it is needed but was also
added in some applications makefiles as the first link option.
The problem is this option is really useful only if added after
the objects or libraries using it (except if using --whole-archive).
And the -lrt options put after were removed to avoid duplicates.

It was resulting in errors linking test application:
eal_timer.c:(.text+0x128): undefined reference to `clock_gettime'
eal_timer.c:(.text+0x166): undefined reference to `clock_gettime'
eal_alarm.c:(.text+0xda): undefined reference to `clock_gettime'
eal_alarm.c:(.text+0x211): undefined reference to `clock_gettime'

It is fixed by removing superfluous -lrt in app makefiles.

Fixes: 281948b4753e ("mk: fix missing librt dependencies")
Fixes: 2f6414f4baf1 ("mk: fix static link with glibc < 2.17")

Reported-by: Piotr Azarewicz <piotrx.t.azarewicz@intel.com>
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>