[Test] Build only enabled LTP tests

[hukoyu]

Fixes #811

With this PR we will build only enabled LTP tests. Not enabled syscall folders are determined at run time by looking at enabled ltp tests files and ltp/testcases/kernel/syscalls folders. If a syscall folder doesn't have any enabled LTP test(s), this folder will be skipped during build and will be reported in skipped binaries and folders statistics.

Also improved the statistics reporting for syscall folders and test binaries.
In addition to Success, Failed status we have Skipped status as well for each syscall folder.

The current results stats is like that:

Syscalls/Folders => Total: 344, Success: 237, Fail: 1, Skip: 106
Tests/Binaries   => Total: 1046, Success: 774, Fail: 2, Skip: 270

Generated 774/1046 binaries in 237/344 folders in /ltp/testcases/kernel/syscalls
Skipped 270/1046 binaries in 106/344 folders since LTP tests not enabled
Failed to generate 2/1046 binaries in 1/344 folders

Here is the complete build log:
ltp_build_log.txt

The only two failures are:
[Test #255/359] Building /ltp/testcases/kernel/syscalls/ptrace Success
WARNING !! ./ptrace06 is not generated
WARNING !! ./simple_tracer is not generated
These two tests are not enabled and Makefile of ptrace returns 0 evenif these two binaries not generated. (There are enabled tests in this syscall folder, that's why it is built)

[davidchisnall]

Looks good to me, but @letmaik / @SeanTAllen are more familiar with this bit of build system than me.

[hukoyu]

The fix working as expected. The a few faliures are shutdown timeout related random failures that @prp 's PR #788 will fix.