busybox: don't clobber dangling symlinks

We sometimes create dangling symlinks in the target directory. That is
because we need canonical targets, as relative targets don't work well
with BR2_ROOTFS_MERGED_USR. For example, the vim package installs the
/bin/vi symlink to /usr/bin/vim. This symlink might be dangling when the
build host has no vim installed there.

Patch the busybox install.sh script to avoid clobber of dangling
symlinks.

Fixes:
http://autobuild.buildroot.net/results/796/796107430db6545401d9926e84f19eaf2040b756/



Cc: Adam Duskett <aduskett@gmail.com>
Cc: Carlos Santos <casantos@datacom.com.br>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>