system: do not overwrite /bin/sh Busybox symlink

system: do not overwrite /bin/sh Busybox symlink

The BR2_SYSTEM_BIN_SH hidden option defines to what binary the /bin/sh
symlinks should point to. If busybox is chosen, then /bin/sh is created
to point to /bin/busybox.

This works fine with the default installation mode of Busybox, but it
fails with the upcoming "individual binaries" mode, in which each applet
is installed as its own binary, and /bin/busybox doesn't exist: we get
/bin/sh as a broken symlink to /bin/busybox.

Since Busybox already installs its own /bin/sh symlink, properly
pointing to /bin/ash or /bin/hush depending on the selected shell, it
doesn't make sense for the BR2_SYSTEM_BIN_SH logic to override
this. Just let Busybox install its own /bin/sh by making
BR2_SYSTEM_BIN_SH empty when Busybox shell is selected as /bin/sh.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
(cherry picked from commit fc91501e6b32ef1b015ac9b0fbf6d64aa1ce0162)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>