tstools: disable parallel build

Commit 0e8cbd59 (tstools: fix build failure in parallel (-j) build) didn't
fix, apparently, the parallel build issue. Revert this commit, and just
disable parallel build.

Fixes:
http://autobuild.buildroot.net/results/b13/b13d31a8a915a740386befb018f859d3df5e9c35/
http://autobuild.buildroot.net/results/287/287249ce9d337d4efc587f833f811851cafc607a/
http://autobuild.buildroot.net/results/e4e/e4ee0ae40ac9d58b3182db52d5a39e6ac3692f60/



Cc: Tzu-Jung Lee <roylee17@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>