common, itest: pass u-boot env variables to itest.s

compare two U-Boot Environment variables with itest.s, example:

=> print tmp ver
tmp=U-Boot 2013.10-g75e
ver=U-Boot 2013.10-g75eb4bc (Jan 21 2014 - 10:35:39)MPC83XX
=> print check_ub_ver
check_ub_ver=if itest.s \${tmp} == \${ver}; then echo equal; else echo diff ;fi
=> run check_ub_ver
diff
=> setenv tmp U-Boot 2013.10-g75eb4bc (Jan 21 2014 - 10:35:39)MPC83XX
=> print tmp ver
tmp=U-Boot 2013.10-g75eb4bc (Jan 21 2014 - 10:35:39)MPC83XX
ver=U-Boot 2013.10-g75eb4bc (Jan 21 2014 - 10:35:39)MPC83XX
=> run check_ub_ver
equal

Signed-off-by: Heiko Schocher <hs@denx.de>