fcb_settings: Use weak function to get storage flash area

On MCU with bank swap capabilities, the offset of the storage area is
not the same before and after a bank swap. This commit introduce a weak
function which returns the default flash area of the storage partition.
On MCU with the bank swap capability the user can define its own
function to get the proper flash area depending on which bank the fw is
run from.
This commit is a workaround for #47732

Signed-off-by: Nicolas VINCENT <nicolas.vincent@vossloh.com>