busybox: add support for CONFIG_FEATURE_INDIVIDUAL

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.

[YOCTO #4570]

(From OE-Core rev: 87efa83191f0b4f4cec3a821b6dfe425c781c077)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>