Fix #206: allow to customize utilities from configure

`ethercatctl` calls some utilites actually hardcoded in the script.
Provide a way to customize them from configure.

The default values are choosen to be backward compatible with the old
behavior, but more sensible values would be preferred, e.g.:

OPTION      ACTUAL DEFAULT      PREFERRED DEFAULT
kmod-dir    /sbin               $sbindir
ip-cmd      /bin/ip             $sbindir/ip

In a major future release this can be changed.

script/Makefile.am

@@ -68,6 +68,8 @@ endif
 # Any precious variable used inside script_templates should appear here
 expand_script = $(SED) \
 	-e 's,[@]VERSION[@],$(VERSION),g' \
+	-e 's,[@]ipcmd[@],$(ipcmd),g' \
+	-e 's,[@]kmoddir[@],$(kmoddir),g' \
 	-e 's,[@]bindir[@],$(bindir),g' \
 	-e 's,[@]sbindir[@],$(sbindir),g' \
 	-e 's,[@]sysconfdir[@],$(sysconfdir),g'

script/ethercatctl.in

@@ -25,13 +25,13 @@
 #
 #------------------------------------------------------------------------------
 
-LSMOD=/sbin/lsmod
-MODPROBE=/sbin/modprobe
-RMMOD=/sbin/rmmod
-MODINFO=/sbin/modinfo
-IP=/bin/ip
+LSMOD="@kmoddir@/lsmod"
+MODPROBE="@kmoddir@/modprobe"
+RMMOD="@kmoddir@/rmmod"
+MODINFO="@kmoddir@/modinfo"
+IP="@ipcmd@"
 
-ETHERCAT=@bindir@/ethercat
+ETHERCAT="@bindir@/ethercat"
 
 #------------------------------------------------------------------------------
 
@@ -39,7 +39,7 @@ if [ "$1" = "-c" ]; then
     ETHERCAT_CONFIG="$2"
     COMMAND="$3"
 else
-    ETHERCAT_CONFIG=@sysconfdir@/ethercat.conf
+    ETHERCAT_CONFIG="@sysconfdir@/ethercat.conf"
     COMMAND="$1"
 fi