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>