Update bootstrap.cfg immediately after interactive prompts

This improves the readablility of the scripts. It also helps avoid
cases where a variable could be set twice in bootstrap.cfg, such as
the DISK variable.

sysb/run.sh

@@ -53,14 +53,14 @@ ask_disk() {
 if [ -z "${DISK}" ] || ! [ -e "/dev/${DISK}" ]; then
     echo "You did not provide a valid disk in the configuration file."
     ask_disk
+
+    echo "export DISK=${DISK}" >> /usr/src/bootstrap.cfg
 fi
 
 PREFIX=/usr
 SOURCES="${PREFIX}/src"
 SYSC=/sysc
 
-echo "export DISK=${DISK}" >> /usr/src/bootstrap.cfg
-
 # Otherwise, add stuff from sysa to sysb
 echo "Mounting sysc"
 mkdir /sysc