efi_loader: correctly support parameter delta in Blt

In the Blt service of the EFI_GRAPHICS_OUTPUT_PROTOCOL the parameter delta
is measured in bytes and not in pixels.

The coding only supports delta being a multiple of four. The UEFI
specification does not explicitly require this but as pixels have a size of
four bytes we should be able to assume four byte alignment.

The corresponding unit test is corrected, too. It can be launched with

	setenv efi_selftest block image transfer
	bootefi selftest

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>