expr_parser: fix issue with hex values in environment

Hex mumeric values directly in the expression were cast correctly
via the t_HEX rule, but ValueErrors would occur if a hex value was
looked up due to the expansion of some environment symbol.

Change-Id: Ia98dfea91eff4ed95778922d38d2967284f4e31b
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>