cmd_nvedit: use explicit typecast for printf

This patch fixes warnings in MAKEALL for avr32:

---8<---
cmd_nvedit.c: In function 'do_env_export':
cmd_nvedit.c:663: warning: format '%zX' expects type 'size_t', but argument 3 has type 'ssize_t'
--->8---

Signed-off-by: Andreas Biemann <biessmann@corscience.de>