Expansion of the (admittedly perverse) "aaa}bbb${ccc" will currently
fail because expand_command() does not check that the closing "}"
occurs later than the opening "${".

Fix by ensuring that the most recent opening "${" is used to match
against the first *subsequent* closing "}".

Total cost of this change: -12 bytes, bringing the overall cost of
this feature to -4 bytes.

Signed-off-by: Michael Brown <mcb30@ipxe.org>