efi_loader: fix setting PlatformLang

The UEFI variables PlatformLang and PlatformLangCodes specify the current
firmware language and the list of all available languages.

Currently their values are hard coded. With the patch a new configuration
variable EFI_PLATFORM_LANG_CODES is provided.

When initializing the UEFI subsystem this configuration variable is used to
initialize PlatformLangCodes. The value of variable PlatformLang is read.
If it is not set, the first language specified in EFI_PLATFORM_LANG_CODES
is used to initialize PlatformLang.

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