grub-efi.bbclass: Fix path in startup.nsh for iso image.

grub-efi.bbclass: Fix path in startup.nsh for iso image.

The path in startup.nsh for iso image is corrupted as follows:
fs0:\EFI\BOOT^Hootx64.efi

Using printf will emit correct path which is:
fs0:\EFI\BOOT\bootx64.efi

This happens because of echo command. Switching to printf
like the one used in efi_populate() function.

Signed-off-by: Pranav Tipnis <pranav.tipnis@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>