qemu robot: fix comments
qemu robot: fix comments

Target should be qemuarm, and added a comment about building sysroots.

Signed-off-by: Nan Zhou <aeb52885e1477134543deb7d235177673b6b0cfe@gmail.com>
Change-Id: I5737c61b51ba38d7db96f18d2f400c43e2ef911d

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts