discover/grub2: Fix handling of positional parameters
Positional parameters are set in the environment with '$' prepended to
the name. This causes lookups to fail because parameter lookups don't
include the '$'.
TESTED:
Added a test that covers positional parameters in GRUB2 parser.
Build succeeds, tests pass.
Bootstrapped-by: Nancy Yuen <yuenn@google.com> Signed-off-by: Alan Dunn <amdunn@google.com> Signed-off-by: Sam Mendoza-Jonas <sam@mendozajonas.com>