Add new system for startup.

Now instead of init() and main() being responsible for all
initialization, individual modules can link in their own
initialization routines.

There are 7 levels for these initializiation routines.

So far these are how the levels are defined

level 0 - Here the world is dark. Nothing is initialized.
          This level is responsible for initializing the
          system clock.

level 1 - The system clock has been configured, but nothing else. Not
          even global variables. This level is responsible for loading
          the data sections from flash and clearing the .bss section.

level 2 - USART2 is enabled and set to be the main kernel logging
          vehicle. From this point on klogf(...) can be used.

level 3 - The NVIC is reset to point to the flash. From this point
          on interrupts can be received. I expect this is where
          most core initialization routines will take place

levels 4 to 7 - User initializiation levels.

main - main() is called after all 8 initialization levels have executed,
       so in a sense main() is like a 9th initialization level, except
       that there is can be only one main() routine whereas there can be
       multiple initalization routines per level.

1 files changed, 2 insertions, 0 deletions

diff --git a/02-usart/include/arch/stm32l4xxx/peripherals/clock.h b/02-usart/include/arch/stm32l4xxx/peripherals/clock.h
index 6e461de..6f628fd 100644
--- a/02-usart/include/arch/stm32l4xxx/peripherals/clock.h
+++ b/02-usart/include/arch/stm32l4xxx/peripherals/clock.h
@@ -123,4 +123,6 @@ int configure_pll(
     pllm_divisor_t pllm, /* PLL denominator. */
     pll_src_t pllsrc /* PLL source */);
 
+uint8_t get_clock_mhz();
+
 #endif /* CORE_CLOCK_H__ */