Add b2qt developer build for qtbase

Because we had enabled this for qtdeclarative before.

Change-Id: I1efd438ebf51b2e632d3f8f39bd707e7eb5a282e
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@qt.io>

coin/platform_configs/qtbase.txt

+Template                          Target OS           Target arch Compiler    Features
+--------------------------------- ------------------- ----------- ----------- -------------------------------------------------------------------------------------
+qtci-linux-Ubuntu-16.04-x86_64-1  QEMU                armv7       GCC         DeveloperBuild, DisableTests
+qtci-linux-Ubuntu-16.04-x86_64-1  QEMU                arm64       GCC         DeveloperBuild, DisableTests