Entering a bootloader (ST system bootloader, or custom mboot) from software
by directly branching to it is not reliable, and the reliability of it
working can depend on the peripherals that were enabled by the application
code. It's also not possible to branch to a bootloader if the WDT is
enabled (unless the bootloader has specific provisions to feed the WDT).
This patch changes the way a bootloader is entered from software by first
doing a complete system reset, then branching to the desired bootloader
early on in the start-up process. The top two words of RAM (of the stack)
are reserved to store flags indicating that the bootloader should be
entered after a reset.
Previously the end of the heap was the start (lowest address) of the stack.
With the changes in this commit these addresses are now independent,
allowing a board to place the heap and stack in separate locations.
This patch allows a particular board to independently specify the linker
scripts for 1) the MCU memory layout; 2) how the different firmware
sections are arranged in memory. Right now all boards follow the same
layout with two separate firmware section, one for the ISR and one for the
text and data. This leaves room for storage (filesystem data) to live
between the firmware sections.
The idea with this patch is to accommodate boards that don't have internal
flash storage and only need to have one continuous firmware section. Thus
the common.ld script is renamed to common_ifs.ld to make explicit that it
is used for cases where the board has internal flash storage.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
