package/dropbear: fix when readlink is from busybox

Busybox "readlink -f" does not canonicalise paths when the target is
missing, while coreutils do.

Fix that by:
  - making an absolute symlink
  - dropping "-f" when calling readlink

Fixes #8276.

Reported-by: Jason Tang <tang@jtang.org>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Paul Cercueil <paul@crapouillou.net>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Tested-by: Jason Tang <tang@jtang.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/dropbear/S50dropbear

@@ -18,7 +18,7 @@ start() {
 	#   - the filesystem is RW (i.e. we can rm the symlink),
 	#     replace the symlink with an actual directory
 	if [ -L /etc/dropbear \
-	     -a "$(readlink -f /etc/dropbear)" = "/var/run/dropbear" ]
+	     -a "$(readlink /etc/dropbear)" = "/var/run/dropbear" ]
 	then
 		if rm -f /etc/dropbear; then
 			mkdir -p /etc/dropbear

package/dropbear/dropbear.mk

@@ -93,7 +93,7 @@ define DROPBEAR_INSTALL_TARGET_CMDS
 	for f in $(DROPBEAR_TARGET_BINS); do \
 		ln -snf ../sbin/dropbear $(TARGET_DIR)/usr/bin/$$f ; \
 	done
-	ln -snf ../var/run/dropbear $(TARGET_DIR)/etc/dropbear
+	ln -snf /var/run/dropbear $(TARGET_DIR)/etc/dropbear
 endef
 
 $(eval $(autotools-package))

package/dropbear/dropbear.service

@@ -10,7 +10,7 @@ After=syslog.target network.target auditd.service
 #     replace the symlink with an actual directory
 ExecStartPre=/bin/sh -c '\
 if [ -L /etc/dropbear \
-     -a "$(readlink -f /etc/dropbear)" = "/var/run/dropbear" ]; then \
+     -a "$(readlink /etc/dropbear)" = "/var/run/dropbear" ]; then \
     if rm -f /etc/dropbear; then \
         mkdir -p /etc/dropbear; \
     else \