Loader improvements

[andreagilardoni]

This PR aims to:

• add a check on header fields to ensure endianness through byteorder zephyr library
• redefine struct sketch_header_v1 to a union which can simplify the procedure of converting a 16 byte buffer into a C struct and respect the strict aliasing rule.

[pillo79]

LGTM, not sure about the need for endianness checks as it would require a very big effort to go over the whole project (code and tools) to make sure there is proper support for it.

EDIT: please add a short commit message that explains why this is useful.

[andreagilardoni]

@pillo79 I believe that the endianness commit has little to no cost in terms of code space (on little endian platforms) and in a distant future where we may try a big endian platform we have one place less to check to make it compatible. I don't see any other reasons except that this is a free improvement in platform independent code.

[leonardocavagnis]

Suggested change

	if (sketch_hdr.ver != 0x1 || sketch_hdr.magic != 0x2341) {
	if (sketch_hdr.ver != SKETCH_VER || sketch_hdr.magic != SKETCH_MAGIC) {

Consider replacing the magic numbers with #define constants for maintainability

[leonardocavagnis]

LGTM :)
minor change suggested

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