The main() function has a predefined type in C which is not so useful for
embedded contexts. This patch renames main() to stm32_main() so we can
define our own type signature for this function. The type signature is
defined to have a single argument which is the "reset_mode" and is passed
through as r0 from Reset_Handler. This allows, for example, a bootloader
to pass through information into the main application.
The Reset_Handler needs to copy the data section and zero the BSS, and
these operations should be as optimised as possible to reduce start up
time. The versions provided in this patch are about 2x faster (on a Cortex
M4) than the previous implementations.
