This adds a layer of convenience abstraction over classical Harvard
architectures (like most AVRs) that do not map the flash memory into
the data address space and modern Harvard architectures or von-Neumann
architectures that do so. The motivation is to safe a lot of RAM for
AVR by storing constant strings into flash.
Our AVR port doesn't make use of an ISR stack and just victimizes the
stack of whatever thread happens to be running, which in most cases is
the idle thread. Hence, the idle stack has to be large enough to
support the ztimer ISR.
If a timer triggers while the idle thread is running, previously a stack
overflow was triggered. This commit increases the idle threads stack size if
xtimer is used.
