dosfstools: bump to version 3.0.15

[Peter: update comment dependency, use DOSFSTOOLS_LDFLAGS for all flags]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Merge branch 'master' of git://git.buildroot.net/buildroot