Re-enable loader efi http boot and fix dv_open bug if dv_init failed

The code in efihttp.c was assuming that dv_open wouldn't be called if
dv_init failed. But the dv_init return value is currently ignored.

Add a new variable, efihttp_init_done and only proceed in dv_open if
it's true. This fixes the loader on systems without efi http support.