cmake: hex: deprecate use of to_hex() and from_hex() utility functions

Since v3.13, CMake has supported math(... OUTPUT_FORMAT <format>) and 0x
prefixes on numbers, which together provides the same functionality as
to_hex() and from_hex() that we've previously maintained.

Users should switch to using math() instead, and our functions are now
marked as deprecated.

Signed-off-by: Attie Grande <attie.grande@argentum-systems.co.uk>