Allow '+' char in var flag names

This fixes an issue seen in [qemu.inc in oe-core](http://git.openembedded.org/openembedded-core/tree/meta/recipes-devtools/qemu/qemu.inc#n94), where a PACKAGECONFIG for gtk+ shows up as a syntax error. This is a minimal change to fix this issue, I don't know if + should be valid in other contexts as well.