gen-options.awk: Work around busybox regex bug.

It appears that busybox awk does not recognize \] in a character class,
interpreting the backslash literally and taking the ] as the end of the
character class:

  % printf '%s\n' ']' '\]' | busybox awk '/^[\]]$/ { print; }'
  \]

  % printf '%s\n' ']' '\]' | gawk '/^[\]]$/ { print; }'
  ]

Busybox does accept []] as meaning "] is part of the character class",
but this syntax is not portable to heirloom (and presumably also Solaris
10) awk, which only works with [\]].  GNU awk accepts either form.

So instead, let's avoid this problematic construct entirely by writing
the test in a different way.