The CMake macro `mbed_post_build_psoc6_merge_hex()` takes the name of
a Cypress target and an optional Cortex-M0 hex image as arguments. The
proper way to define and parse optional arguments of a function or
macro is `cmake_parse_arguments()`, which allows the caller to
indicate what they are passing rather than rely on an argument's
relative position within `${ARGN}` which is not rigorous.

Also, avoid duplicating the common part of the post build command
when the optional argument is passed/not passed.