package/dosfstools: add missing symlinks

Install alternative symlinks for dosfstools mkfs.fat, fsck.fat and
fatlabel. Those symlinks are generally installed by make install, but
buildroot does not use make install for dosfstools, but symlinks should
be installed anyway.

[Thomas: rewrite the installation logic to use make style code rather
than shell code.]

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>