package/micropython: define MICROPY_NLR_SETJMP for xtensa

As suggested on https://github.com/micropython/micropython/issues/6551
define MICROPY_NLR_SETJMP to avoid the xtensa specific implementation
of nlr_push.

Fixes:
- http://autobuild.buildroot.net/results/5fc8669b5c768ccfc02bd20d1159bce7fe43683e



Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>