bzip2: don't overwrite busybox during installation

The bzip2 make install target follows symlinks, so if busybox is
enabled and the bzip/bunzip2 applets are installed, bin/busybox will
get overwritten by bzip2.

Fix it by ensuring any previously installed bzip2/bunzip2/bzcat (the
applets available in busybox) are removed before running make install.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>