efi_selftest: testing read-only variables

efi_selftest: testing read-only variables

The UEFI spec has this sentence:
"Variables that have runtime access but that are not nonvolatile are
read-only data variables once ExitBootServices() is performed."

At least EDK2 therefore treats variable PlatformLangCodes only as read-only
at runtime. Nowhere do we make this variable read-only in U-Boot.

Do not use variable PlatformLangCodes for testing if the read only
attribute of variables is observed before ExitBootServices().

Fixes: 5a24239c951e ("efi_loader: selftest: enable APPEND_WRITE tests")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>