stm32/desig: fix/cleanup desig_get_unique_id and to string functionality

This was inspired by an Arch Linux provided ARM GCC 5.3.0 bug:
It gave an
"internal compiler error: in expand_expr_addr_expr_1, at expr.c:7736"
with the array version of the desig_get_unique_id.

While I was at it, fixed:
- a potential alignment issue with casting uint8_t* buf to uint32_t*
- a funny static in the string definition that does nothing (given const)

include/libopencm3/stm32/desig.h

@@ -41,7 +41,7 @@ uint16_t desig_get_flash_size(void);
  * Note: ST specifies that bits 31..16 are _also_ reserved for future use
  * @param result pointer to at least 3xuint32_ts (96 bits)
  */
-void desig_get_unique_id(uint32_t result[]);
+void desig_get_unique_id(uint32_t *result);
 
 /**
  * Read the full 96 bit unique identifier and return it as a