release: Remove C-like string comparison operator

Per [(1), i.e. test(1), the string comparison operator should be =,
not == in sh(1) scripts.

No functional change. FreeBSD's test accepts == as an alias for =
for compatibility with gnu test's == extension.

Reviewed by: imp@, gjb@
Pull Request: https://github.com/freebsd/freebsd-src/pull/485