busybox: add support for CONFIG_FEATURE_INDIVIDUAL
Previously, if CONFIG_FEATURE_INDIVIDUAL was enabled for busybox,
yocto-based systems could start correctly.
This is because if busybox is built as individual apps, '/bin/busybox'
may not be present, so setting the default ALTERNATIVE_TARGET to
'/bin/busybox' is not appropriate and could lead to errors.
This patch fixes this problem by checking the existence of '/bin/busybox'
before setting the ALTERNATIVE_TARGET to '/bin/busybox'.
After this change, if busybox is built as individual apps, we'll have
links like '/bin/ls -> /bin/ls.busybox', otherwise, we'll have links
like '/bin/ls -> /bin/busybox'.
Note there's a grep expression change in this patch. The old expression
doesn't work well, it has an unwanted underscore, so I changed it to make
it work.
