all: Make file-scope variables internally linked where applicable (#57)

Narrows the scope of visible symbols to where they're actually used. Also
makes it easier to see true globals in source files
(ones used from multiple translation units)

exosphere/src/configitem.c

@@ -8,7 +8,7 @@
 #include "fuse.h"
 #include "utils.h"
 
-int g_battery_profile = 0;
+static int g_battery_profile = 0;
 
 uint32_t configitem_set(enum ConfigItem item, uint64_t value) {
     if (item != CONFIGITEM_BATTERYPROFILE) {