firmware-utils: replace md5 code with Alexander Peslyak's implementation

firmware-utils: replace md5 code with Alexander Peslyak's implementation

Our current implementation is pretty old and uses some pre-standard/old
ANSI C style that triggers warnings like:
warning: call to function 'MD5_Init' without a real prototype [-Wunprototyped-calls]

This is caused by declarations specified in a following way:
src/md5.h:60:6: note: 'MD5_Init' was declared here
 void MD5_Init ();

Having these warnings makes it harded to notice real problems. We could
try hiding them but it makes more sense to just use a cleaner code.
Another tiny gain from this switch is slightly reduced binary size, on
x86_64 tplink-safeloader's size 48104 became 48003.

The new code is public domain, uses "heavily cut-down BSD license".

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>