Workaround: Implement g_nvic_prio_bits for disabled.c

When __uvisor_mode is UVISOR_DISABLED the NVIC APIs are used instead of
the vIRQ ones. Now that those APIs are pre-compiled into a library, the
NVIC APIs use the uVisor built-in definitions from cmsis/core_generic.h.

In that file __NVIC_PRIO_BITS is re-defined as a global variable which
is evaluated at start-up. This behaviour has now been mirrored to the
case of disabled uVisor.

Warning: This is a workaround.

Applications that are written following the uVisor guidelines for the
use of vIRQ APIs will work without problems:
> The vIRQ_SetVector must be used before any other vIRQ API for any given
  IRQn slot.

While this is currently the case for all mbed OS code, it is not
guaranteed to work with new code. For example, if vIRQ_SetPriority is
called before vIRQ_SetVector unpredictable behaviour will follow.

This issue will be fixed by a more generalized approach to disabled
uVisor.

Author: AlessandroA

api/src/disabled.c

@@ -21,6 +21,9 @@
 #include <stdint.h>
 #include <string.h>
 
+/* Number of implemented priority bits */
+uint8_t g_nvic_prio_bits;
+
 /* Symbols exported by the mbed linker script */
 UVISOR_EXTERN uint32_t __uvisor_cfgtbl_ptr_start;
 UVISOR_EXTERN uint32_t __uvisor_cfgtbl_ptr_end;
@@ -186,7 +189,8 @@ static void uvisor_disabled_default_vector(void)
 
 void uvisor_disabled_set_vector(uint32_t irqn, uint32_t vector)
 {
-    uint8_t box_id;
+    uint8_t prio_bits, box_id;
+    uint8_t volatile *prio;
 
     /* Check IRQn.
      * We only allow user IRQs to be registered (NVIC). This is consistent with
@@ -204,6 +208,17 @@ void uvisor_disabled_set_vector(uint32_t irqn, uint32_t vector)
      * user vectors explicitly set using this API are registered in the table. */
     if (SCB->VTOR != (uint32_t) g_irq_table) {
         SCB->VTOR = (uint32_t) g_irq_table;
+
+        /* Detect the number of implemented priority bits.
+         * The architecture specifies that unused/not implemented bits in the
+         * NVIC IP registers read back as 0. */
+        __disable_irq();
+        prio = (uint8_t volatile *) &(NVIC->IP[0]);
+        prio_bits = *prio;
+        *prio = 0xFFU;
+        g_nvic_prio_bits = (uint8_t) __builtin_popcount(*prio);
+        *prio = prio_bits;
+        __enable_irq();
     }
 
     /* Register IRQ.