coreutils: get the uptime program to work

coreutils configure script needs to know whether /proc/uptime is
available, but this is not possible in a cross-compilation
context. This leads to an uptime program that fails to work on the
target, as it has been compiled without /proc/uptime reading support.

This commit fixes that by telling coreutils at configure time that
/proc/uptime will be available on the target (which seems to be a
reasonable assumption on Linux systems).

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