Restore automatic selection between busybox and klibc by default

Some systems are working alright with the old default of BUSYBOX=y and
no busybox installed.  We should only force use of busybox if another
package installing initramfs scripts needs it.

Change the default from BUSYBOX=y to BUSYBOX=auto, and give that the
old semantics.

Closes: #809204
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

conf/initramfs.conf

@@ -20,12 +20,14 @@
 MODULES=most
 
 #
-# BUSYBOX: [ y | n ]
+# BUSYBOX: [ y | n | auto ]
 #
 # Use busybox shell and utilities.  If set to n, klibc utilities will be used.
+# If set to auto (or unset), busybox will be used if installed and klibc will
+# be used otherwise.
 #
 
-BUSYBOX=y
+BUSYBOX=auto
 
 #
 # KEYMAP: [ y | n ]

hooks/busybox

@@ -16,14 +16,19 @@ prereqs)
 esac
 
 # busybox
-if [ "${BUSYBOX}" != "n" ]; then
-	if ! [ -e ${BUSYBOXDIR}/busybox ]; then
+if [ "${BUSYBOX}" = "n" ]; then
+	exit 0
+fi
+if ! [ -e ${BUSYBOXDIR}/busybox ]; then
+	if [ "${BUSYBOX}" = "y" ]; then
 		echo >&2 "E: busybox is required but not installed"
 		exit 1
+	else
+		exit 0
 	fi
-	. /usr/share/initramfs-tools/hook-functions
-	rm -f ${DESTDIR}/bin/sh
-	rm -f ${DESTDIR}/bin/busybox
-	copy_exec ${BUSYBOXDIR}/busybox /bin/busybox
-	ln -s busybox ${DESTDIR}/bin/sh
 fi
+. /usr/share/initramfs-tools/hook-functions
+rm -f ${DESTDIR}/bin/sh
+rm -f ${DESTDIR}/bin/busybox
+copy_exec ${BUSYBOXDIR}/busybox /bin/busybox
+ln -s busybox ${DESTDIR}/bin/sh

hooks/klibc

@@ -19,7 +19,8 @@ esac
 cp -pnL /usr/lib/klibc/bin/* ${DESTDIR}/bin
 cp -pL /lib/klibc-*.so ${DESTDIR}/lib
 rm -f ${DESTDIR}/bin/kinit* ${DESTDIR}/bin/zcat
-if [ "${BUSYBOX}" = "n" ]; then
+if [ "${BUSYBOX}" = "n" ] || \
+   { [ "${BUSYBOX}" != "y" ] && ! [ -e ${BUSYBOXDIR}/busybox ]; }; then
 	if [ -e ${DESTDIR}/bin/sh.shared ]; then
 		# Some platforms build a shared klibc/sh:
 		mv ${DESTDIR}/bin/sh.shared ${DESTDIR}/bin/sh