Add dhex_dump() and DHEXDUMP() to format and print data in hexadecimal

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
OK for the __printf macro. The forward declaration is needed due to -Wmissing-format-attribute which causes:
  core/arch/arm32/kernel/tee_core_trace.c: In function ‘append’:
  core/arch/arm32/kernel/tee_core_trace.c:239:2: error: function might be possible candidate for ‘gnu_printf’ format attribute [-Werror=suggest-attribute=format]
    len = vsnprintf(sbuf->ptr, left, fmt, ap);
    ^
  core/arch/arm32/kernel/tee_core_trace.c:239:2: error: function might be possible candidate for ‘gnu_printf’ format attribute [-Werror=suggest-attribute=format]

Another option is:
cflags-remove-tee_core_trace.c-y := -Wmissing-format-attribute
Which one do you like better?