Drop newlib printf: log via BDK s_vprintf, add debug.ini log switch

log_write was the only vsnprintf user and pulled in newlib's printf,
dtoa, malloc and soft-float code (~44 KB). The payload now fits the
RCM limit again (146800 -> 102801 bytes); the build fails if it grows
past it. debug.ini log=1 writes sd:/config/omninx/install.log.

source/fs.h

@@ -18,3 +18,4 @@ int folder_delete(const char *path);
 void log_init(const char *path);
 void log_close(void);
 void log_write(const char *fmt, ...);
+bool log_is_enabled(void);