bootscript-uboot: fix test -z cases

"test -z $var" do not work correctly if var is not defined:

u-boot=> pri test
u-boot=> if test -z $test; then echo 123; fi
u-boot=> if test -z "$test"; then echo 123; fi
123
u-boot=>

Signed-off-by: Mikhail Vanyulin <mikhail.vanyulin@rtsoft.de>