Fixes#1749
Defining a global true `bool core1_separate_stack = true` will separate
the two cores' stacks, with core 0 using the scratch RAM while core 1
will use 8K from the heap.
Update pico-sdk to 1.1.2
Add methods to block the opposite core while doing flash updates.
Ensure opposite core is stopped in LittleFS and EEPROM while doing
flash updates.
Update documentation with new calls.
Support running code on the second core by adding a setup1() and/or
a loop1() routine to a sketch. These functions operate exactly like
the normal Arduino ones, and anything they call will be run on
the second core automatically.
Add a simple multicore example.
