package/dosfstools: introduce custom install routine

We can't use dosfstools' install target, because it'll install *all*
binaries, even the disabled ones. Also, we can't just delete dosfstools
binaries from the target directory after installing them, because other
packages (specifically Busybox) may provide tools of the same name, and
we may end up deleting those instead.

To avoid any issues, we create our own install routines, which only
copy the enabled binaries into the target location.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
[Thomas: use full destination path for INSTALL commands.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>