efi_loader: add copyright notice

Some amount of code was moved/derived from efi_variable.c regarding
UEFI secure boot, in particluar in the commit 012c56ac ("efi_loader:
restructure code for TEE variables").
So add the orignal author's copyright notice.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>