efi: pstore: Omit efivars caching EFI varstore access layer

Avoid the efivars layer and simply call the newly introduced EFI
varstore helpers instead. This simplifies the code substantially, and
also allows us to remove some hacks in the shared efivars layer that
were added for efi-pstore specifically.

In order to be able to delete the EFI variable associated with a record,
store the UTF-16 name of the variable in the pstore record's priv field.
That way, we don't have to make guesses regarding which variable the
record may have been loaded from.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/Kconfig

@@ -22,6 +22,7 @@ config EFI_ESRT
 config EFI_VARS_PSTORE
 	tristate "Register efivars backend for pstore"
 	depends on PSTORE
+	select UCS2_STRING
 	default y
 	help
 	  Say Y here to enable use efivars as a backend to pstore. This