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.

sysa/run.sh

@@ -71,6 +71,8 @@ if [ -z "${CHROOT}" ]; then
     echo "or not. Is it? (yes/no answer)"
     ask_chroot
     echo
+
+    echo "export CHROOT=${CHROOT}" >> "${SOURCES}/bootstrap.cfg"
 fi
 
 ask_timestamps() {
@@ -91,15 +93,13 @@ if [ -z "${FORCE_TIMESTAMPS}" ]; then
     echo "fully reproducible disk image. (yes/no answer)"
     ask_timestamps
     echo
+
+    echo "export FORCE_TIMESTAMPS=${FORCE_TIMESTAMPS}" >> "${SOURCES}/bootstrap.cfg"
 fi
 
 echo "Thank you! All done."
 
-# Write to bootstrap.cfg
-rm "${SOURCES}/bootstrap.cfg"
-for var in CHROOT FORCE_TIMESTAMPS DISK ARCH UPDATE_CHECKSUMS; do
-    echo "export ${var}=${!var}" >> "${SOURCES}/bootstrap.cfg"
-done
+echo "export ARCH=${ARCH}" >> "${SOURCES}/bootstrap.cfg"
 
 mkdir -p "${DESTDIR}" "${REPO}" /dev
 

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