build: Move optimization flags into a separate variable

Isolate optimization related compiler flags from CFLAGS and move them into a
separate COPTS variable so that it is easier to override optimizations from
the environment.

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

--- a/pppd/plugins/radius/Makefile.linux
+++ b/pppd/plugins/radius/Makefile.linux
@@ -12,7 +12,8 @@ VERSION = $(shell awk -F '"' '/VERSION/
 INSTALL	= install
 
 PLUGIN=radius.so radattr.so radrealms.so
-CFLAGS=-I. -I../.. -I../../../include -O2 -fPIC -DRC_LOG_FACILITY=LOG_DAEMON
+COPTS = -O2
+CFLAGS=-I. -I../.. -I../../../include $(COPTS) -fPIC -DRC_LOG_FACILITY=LOG_DAEMON
 
 # Uncomment the next line to include support for Microsoft's
 # MS-CHAP authentication protocol.
--- a/pppdump/Makefile.linux
+++ b/pppdump/Makefile.linux
@@ -2,7 +2,8 @@ DESTDIR = $(INSTROOT)@DESTDIR@
 BINDIR = $(DESTDIR)/sbin
 MANDIR = $(DESTDIR)/share/man/man8
 
-CFLAGS= -O -I../include/net
+COPTS = -O
+CFLAGS= $(COPTS) -I../include/net
 OBJS = pppdump.o bsd-comp.o deflate.o zlib.o
 
 INSTALL= install