The softethervpn does not actually supported CCFLAGS, or other standard
variables, so we need to override CC to include all of those to fix
build errors with external toolchains.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

- bugfix #3629
- readded nossl3 patch

Signed-Off-by: Federico Di Marco <fededim@gmail.com>

- bugfix #3629

Signed-Off-by: Federico Di Marco <fededim@gmail.com>

- based on the work done by el1n with his authorization (https://github.com/el1n/OpenWRT-package-softether)
- updated softethervpn to latest version
- introduced necessary patches for lede compatibility (nossl3)

Signed-Off-by: Federico Di Marco <fededim@gmail.com>