efivars: Use string functions in pstore_write

Instead of open-coding the string operations for comparing the prefix of
the variable names, use the provided utf16_* string functions.

This patch also changes the calls to efi.set_variable to
efivars->ops->set_variable so that the right function gets called in the
case of gsmi (which doesn't have a valid efi structure).

As well, make sure that we only consider variables with the right vendor
string.

Signed-off-by: Mike Waychison <mikew@google.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>