tinc: new package

[Thomas:
 - fix license from GPLv2 to 'GPLv2+ with OpenSSL exception'
 - add -std=c99 to CFLAGS, otherwise it doesn't build with a minimal
   toolchain configuration
 - add BR2_USE_MMU dependency since fork() is used.]

Signed-off-by: Zoltan Gyarmati <mr.zoltan.gyarmati@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>