Allow evaluation of arbitrarily-positioned arguments in GRUB2 parser

Allow evaluation of arbitrarily-positioned arguments in GRUB2 parser

GRUB2 allows essentially arbitrary numbers of positional arguments, so
ensure that they can be evaluated within scripts.  GRUB2 also appears
to support arbitrary numbers of leading 0's in positional parameters
(i.e., $01 should evalute the same as $1), but this doesn't seem like
a particularly important case to support.

Tested:
 Modified test-grub2-pos-param to cover higher-numbered positional
 arguments.

Signed-off-by: Alan Dunn <amdunn@google.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>