x86, efi: Allow slash in file path of initrd

x86, efi: Allow slash in file path of initrd

When initrd file didn't put at the same place with stub kernel, we
need give the file path of initrd, but need use backslash to separate
directory and file. It's not friendly to unix/linux user, and not so
intuitive for bootloader forward paramters to efi stub kernel by
chainloading.

This patch add support to handle_ramdisks for allow slash in file path
of initrd, it convert slash to backlash when parsing path.

In additional, this patch also separates print code of efi_char16_t from
efi_printk, and print out the path/filename of initrd when failed to open
initrd file. It's good for debug and discover typo.

Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Lee, Chun-Yi <jlee@suse.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>