scripts/functions: fix date munging when ns are not available

On some systems (eg. *BSD and Darwin), date does not support nanoseconds
(%N) precision. Instead of printing '%N' in this case, it just prints 'N'.
Fix the sed expression to handle this case.

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

 reporter_comment="Experimental toolchain for ARM cortex-m3 in Thumb-only, noMMU, and newlib."