bootstrap: Port back to old shells (#13199)

Since commit f39e82c9 (bootstrap: Re-implement command line option
processing, 2011-12-16) bootstrap uses POSIX shell expressions of the
form "${x#y}" to remove prefix pattern 'y' from the vaule of 'x'.
Although this is allowed by POSIX old shells on some platforms do not
support it.  Revert to using 'sed' to work with old shells.