Installation: Remove USR_PREFIX from Makefile

This commit removes USR_PREFIX and introduces ETC_PREFIX instead. The
purpose of PREFIX is the installation prefix in the system, not the
installation destination. The latter one is what DESTDIR is used for.

This should clear up the confusion. PREFIX was already mis-used as
installation destination. But that doesn't mix well with how the make
targets are designed.

CC: Timofey Titovets <nefelim4ag@gmail.com>
Signed-off-by: Kai Krakow <kai@kaishome.de>

scripts/beesd.in

@@ -12,7 +12,7 @@ export CONFIG_FILE
 export UUID AL16M
 
 readonly AL16M="$((16*1024*1024))"
-readonly CONFIG_DIR=@PREFIX@/etc/bees/
+readonly CONFIG_DIR=@ETC_PREFIX@/bees/
 
 ## Pre checks
 {

scripts/beesd@.service.in

@@ -5,7 +5,7 @@ After=sysinit.target
 
 [Service]
 Type=simple
-ExecStart=@PREFIX@/usr/sbin/beesd %i
+ExecStart=@PREFIX@/sbin/beesd %i
 Nice=19
 KillMode=control-group
 KillSignal=SIGTERM