configure: Don't use bash-specific string-replacement syntax

The variable string-replacement syntax ${var/old/new} is a bashism
(though it is also supported by some other shells), and for instance
does not work with the NetBSD /bin/sh, which complains:
 ../src/configure: 687: Syntax error: Bad substitution

Replace it with a more portable sed-based approach, similar to
what we already do in quote_sh().

Note that shellcheck also diagnoses this:

In ./configure line 687:
    e=${e/'\'/'\\'}
      ^-----------^ SC2039: In POSIX sh, string replacement is undefined.
           ^-- SC1003: Want to escape a single quote? echo 'This is how it'\''s done'.
                ^-- SC1003: Want to escape a single quote? echo 'This is how it'\''s done'.

In ./configure line 688:
    e=${e/\"/'\"'}
      ^----------^ SC2039: In POSIX sh, string replacement is undefined.

Fixes: 8154f5e6 ("meson: Prefix each element of firmware path")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-id: 20220720152631.450903-4-peter.maydell@linaro.org