This commit uses the REGION_ALIAS GNU linker command to simplify the linker
snippets and consolidate the duplication.
Signed-off-by: Damien George <damien@micropython.org>
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 demonstrates how to use external QSPI flash in XIP (execute in place)
mode. The default configuration has all extmod/ code placed into external
QSPI flash, but other code can easily be put there by modifying the custom
f769_qspi.ld script.
