otasigcheck: Fix the sed statement for multi-digit indices

It appears that some versions of sed do not work with the + symbol.
Instead of checking for one or more digits, check for a digit,
followed by zero or more digits.

Change-Id: I064df6a2bac4a634a3684ac1a5289dca1f4ba29c
Merge remote-tracking branch 'CyanogenMod/cm-11.0' into cm-11.0