fs: add root filesystem global variable

fs/fs.cc

 #include "fs.hh"
 
+filesystem* rootfs;
+
 file::~file()
 {
 }

fs/fs.hh

@@ -17,4 +17,6 @@ public:
     virtual file *open(std::string name) = 0;
 };
 
+extern filesystem* rootfs;
+
 #endif

loader.cc

@@ -48,6 +48,7 @@ int main(int ac, char **av)
     idt.load_on_cpu();
 
     bootfs fs;
+    rootfs = &fs;
     file* f = fs.open("/usr/lib/libjvm.so");
     char buf[100];
     f->read(buf, 0, 100);