[boot] Go back to loading symbol table in boot

The symbol table needs to be passed to the panic handler very early in
the kernel, loading it in init is far less useful. Return it to the boot
directory and remove it from the initrd.

src/boot/bootconfig.h

@@ -31,6 +31,7 @@ public:
     inline const descriptor & kernel() const { return m_kernel; }
     inline const descriptor & init() const { return m_init; }
     inline const wchar_t * initrd() const { return m_initrd; }
+    inline const wchar_t * symbols() const { return m_symbols; }
     inline const descriptors & panics() { return m_panics; }
 
 private:
@@ -39,6 +40,7 @@ private:
     descriptor m_init;
     descriptors m_panics;
     wchar_t const *m_initrd;
+    wchar_t const *m_symbols;
 };
 
 } // namespace boot