boot: image: add a stage pre-load

Add a stage pre-load that could
check or modify an image.

For the moment, only a header with a signature is
supported. This header has the following format:
- magic : 4 bytes
- version : 4 bytes
- header size : 4 bytes
- image size : 4 bytes
- offset image signature : 4 bytes
- flags : 4 bytes
- reserved0 : 4 bytes
- reserved1 : 4 bytes
- sha256 of the image signature : 32 bytes
- signature of the first 64 bytes : n bytes
- image signature : n bytes
- padding : up to header size

The stage uses a node /image/pre-load/sig to
get some informations:
- algo-name (mandatory) : name of the algo used to sign
- padding-name : name of padding used to sign
- signature-size : size of the signature (in the header)
- mandatory : set to yes if this sig is mandatory
- public-key (madatory) : value of the public key

Before running the image, the stage pre-load checks
the signature provided in the header.

This is an initial support, later we could add the
support of:
- ciphering
- uncompressing
- ...

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
diff --git a/boot/Makefile b/boot/Makefile
index 75366c8..1b99e6e 100644
--- a/boot/Makefile
+++ b/boot/Makefile
@@ -22,6 +22,7 @@
 obj-$(CONFIG_$(SPL_TPL_)FIT_SIGNATURE) += fdt_region.o
 obj-$(CONFIG_$(SPL_TPL_)FIT) += image-fit.o
 obj-$(CONFIG_$(SPL_)MULTI_DTB_FIT) += boot_fit.o common_fit.o
+obj-$(CONFIG_$(SPL_TPL_)IMAGE_PRE_LOAD) += image-pre-load.o
 obj-$(CONFIG_$(SPL_TPL_)IMAGE_SIGN_INFO) += image-sig.o
 obj-$(CONFIG_$(SPL_TPL_)FIT_SIGNATURE) += image-fit-sig.o
 obj-$(CONFIG_$(SPL_TPL_)FIT_CIPHER) += image-cipher.o