efi_selftest: add selftests for loadfile2 used to load initramfs

Provide a unit test loading an initial ramdisk using the
EFI_LOAD_FILE2_PROTOCOL. The test is only executed on request.

An example usage - given a file image with a file system in partition 1
holding file initrd - is:

* Configure the sandbox with

  CONFIG_EFI_SELFTEST=y
  CONFIG_EFI_LOAD_FILE2_INITRD=y
  CONFIG_EFI_INITRD_FILESPEC="host 0:1 initrd"

* Run ./u-boot and execute

    host bind 0 image
    setenv efi_selftest load initrd
    bootefi selftest

This would provide a test output like:

    Testing EFI API implementation

    Selected test: 'load initrd'

    Setting up 'load initrd'
    Setting up 'load initrd' succeeded

    Executing 'load initrd'
    Loaded 12378613 bytes
    CRC32 2997478465
    Executing 'load initrd' succeeded

Now the size and CRC32 can be compared to the provided file.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>