turn on -Werror and fix resulting errors

src/systemcmds/mixer/module.mk

@@ -41,3 +41,5 @@ SRCS		 = mixer.cpp
 MODULE_STACKSIZE = 4096
 
 MAXOPTIMIZATION	 = -Os
+
+EXTRACXXFLAGS = -Wno-error

src/systemcmds/mtd/module.mk

@@ -6,3 +6,5 @@ MODULE_COMMAND	 = mtd
 SRCS		 = mtd.c 24xxxx_mtd.c
 
 MAXOPTIMIZATION	 = -Os
+
+EXTRACFLAGS	= -Wno-error

src/systemcmds/param/param.c

@@ -212,7 +212,7 @@ static void
 do_show(const char* search_string)
 {
 	printf(" + = saved, * = unsaved\n");
-	param_foreach(do_show_print, search_string, false);
+	param_foreach(do_show_print, (char *)search_string, false);
 
 	exit(0);
 }

src/systemcmds/tests/module.mk

@@ -34,3 +34,5 @@ SRCS			 = test_adc.c \
 			   test_conv.cpp \
 			   test_mount.c \
 			   test_mtd.c
+
+EXTRACXXFLAGS = -Wno-error