libcap: add menu entry to install tools.

libcap provides a set of tools to manipulate capabilities, among which
`setcap(8)` and `getcap(8)`. A new menu entry has been added to install them.

[Thomas: mark the new option as requiring MMU support, because the
programs use fork(), and completely rework the new .mk logic as it
wasn't taking into account the BR2_PREFER_STATIC_LIB case.]

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>