Fix ConfigureOnly option in build.sh
ConfigureOnly is intended to run the CMake to configure the build, but
nothing else. However, when you passed configureonly to build.sh, it set
__SkipConfigure=1, which made it skip the configure step of the build.
This change fixes that by removing that option from the options set my
configureonly, and adds a check right after configure and right before
running the build to exit if we are only configuring.
Commit migrated from https://github.com/dotnet/coreclr/commit/
a69d5a89a1c018f9179c0f02eca1e995cd52cf9b