Issues with make netstandard on Ubuntu 16.04

[flashfm]

Hi,
I found 2 issues preventing make netstandard working on Ubuntu 16.04:

1. tools/netstandard/CreateSigningKey/Program.cs cannot be compiled
2. Makefile.csharp.mk line

@echo [assembly: AssemblyKeyFile("$(notdir $(CLR_KEYFILE))")] >> $(NETSTANDARD_OBJ_DIR)$SAssemblyInfo.cs 

is wrong because of quote char, I had to add additional quotes and escape the inner quotes.

It seems that pull request #586 targets these issues in particular, as well as #634

[Mizux]

#668 Will provide lot of change, should test and fix it once it is integrated.
I.e. we are moving to dotnet first instead of mono

[Mizux]

using .Net Core SDK 2.1.302 it seems that CreateSigningKey works on all targeted platform (cf tools/docker/*)

There was issue on unix concerning the resolution of CurrentWorkingDirectory() when targeting netcoreapp2.0
cf: issues dotnet/cli#9073, dotnet/cli#9319 and the fix dotnet/cli#9504 when targeting netcoreapp2.1 available with .Net Core SDK >= 2.1.302