Correct getopts flags

[gogolok]

• The option -v does not exist, but -f.

• Prefix the option string with a leading colon so the 'actual' bad flag gets reported.

Before:

./build.sh: illegal option -- w
Invalid option: -
Usage:
  ./build.sh [-f <prefix>] [-p <platform>] <dir>
    -f    prefix to use for images      (default: cnbs/sample-base)
    -p    prefix to use for images      (default: linux/amd64)
   <dir>  directory to build

Now:

Invalid option: -w
Usage:
  ./build.sh [-f <prefix>] [-p <platform>] <dir>
    -f    prefix to use for images      (default: cnbs/sample-base)
    -p    prefix to use for images      (default: linux/amd64)
   <dir>  directory to build

[AidanDelaney]

This PR enabled silent error handling in the getopts shell builtin. This seems reasonable in this context.