[U-Boot] buildman cause PC hang for long time

[Peng Fan]

Hi Simon,

I am adding i.MX8MM/8MN to upstream, but when run 
" ./tools/buildman/buildman aarch64 -x tegra,ls1,ls2,mvebu,uniphier,sunxi,samsung,rockchip,xilinx -j8 "
It cause my host pc response very slow, seems it exhausted my CPU resources.

   aarch64:  w+   hikey
+===================== WARNING ======================
+This board does not use CONFIG_DM_USB. Please update
+the board to use CONFIG_DM_USB before the v2019.07 release.
+Failure to update by the deadline may result in board removal.
+See doc/driver-model/MIGRATION.txt for more info.
+====================================================
   30   23    0 /54     0:00:11  : hikey

However when I build hikey i.mx8mn i.mx8mm standalone, I do not met issue. But when I 
run the upper batch command, when it almost reach the end of the build, my host pc
responses very slow.

Do you have any idea?

Thanks,
Peng.