configure: Allow overriding uname results

In a cross compile setting it makes no sense to rely on the "uname" values
reported by the build host system. This patch allows overriding the
"uname -r", "uname -s" and "uname -m" results with the "UNAME_R", "UNAME_S"
and "UNAME_M" environment variables.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

--- a/configure
+++ b/configure
@@ -8,9 +8,9 @@ SYSCONF=/etc
 #  if [ -d /NextApps ]; then
 #    system="NeXTStep"
 #  else
-  system=`uname -s`
-  release=`uname -r`
-  arch=`uname -m`
+  system=${UNAME_S:-`uname -s`}
+  release=${UNAME_R:-`uname -r`}
+  arch=${UNAME_M:-`uname -m`}
 #  fi
 state="unknown"
 
