Temporary fix for muilticore realloc (#625)

This PR includes a pico-sdk built using the fix in
raspberrypi/pico-sdk#864

To properly fix it requires a new pico-sdk release. but until then I am
building against my own fork of pico-sdk and including the binary.
When 1.3.2 is out (and assuming my PR is approved) I'll update the pico-sdk
link.

Fixes #614

Author: earlephilhower

lib/libpico.a

@@ -127,6 +127,7 @@
 -Wl,--wrap=powf
 -Wl,--wrap=powint
 -Wl,--wrap=powintf
+-Wl,--wrap=realloc
 -Wl,--wrap=remainder
 -Wl,--wrap=remainderf
 -Wl,--wrap=remquo