Avoid linker errors with inline function and -O0 optimization.

GCC reported a linker error because of a missing ihex_fromhex4()
function when compiling without optimization.  Since the function is
only used in ihex_parse.c and declared inline, it may as well be
declared static.
Remove unused variable in test_memory_is_copied_1().
Fix misspelled type name u_int8_t in tests.
Mark unused variables in tests to squelch compiler warnings.

For convenience and compatibility, define a macro UNUSED with the
actual attribute declaration.
Fix missing cast in ihex_fromhex*() tests.

The tested functions expect pointers to uint8_t data, but the given
string literals are by default (const char*), which is signed.
Add missing void argument to function declarations.