configure: Fix evaluation of config-host.mak in create_config

Only match on true dir variable assignments, avoid generating garbage
due to the "# Configured with: ..." line which may contain "*dir=" as
well.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

create_config

@@ -13,7 +13,7 @@ case $line in
     pkgversion=${line#*=}
     echo "#define QEMU_PKGVERSION \"$pkgversion\""
     ;;
- prefix=* | *dir=*) # directory configuration
+ prefix=* | [a-z]*dir=*) # directory configuration
     name=${line%=*}
     value=${line#*=}
     define_name=`echo $name | tr '[:lower:]' '[:upper:]'`