Fix ConfigureOnly option in build.sh

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