qemu robot: fix comments

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

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I5737c61b51ba38d7db96f18d2f400c43e2ef911d
diff --git a/run-qemu-robot-test.sh b/run-qemu-robot-test.sh
index 3c2f407..e40dd4a 100755
--- a/run-qemu-robot-test.sh
+++ b/run-qemu-robot-test.sh
@@ -10,7 +10,7 @@
 #  UPSTREAM_WORKSPACE = The directory from which the QEMU components are being
 #                       imported from. Generally, this is the build directory
 #                       that is generated by the OpenBMC build-setup.sh script
-#                       when run with "target=qemu".
+#                       when run with "target=qemuarm".
 #                       Example: /home/builder/workspace/openbmc-build/build.
 #
 # Optional Variables:
@@ -42,7 +42,8 @@
 #                       QEMU relative to upstream workspace.  Default is
 #                       ./tmp/sysroots/${QEMU_ARCH}/usr/bin/qemu-system-arm
 #                       which is the default location when doing a bitbake
-#                       of obmc-phosphor-image
+#                       of obmc-phosphor-image. If you don't find the sysroots
+#                       folder, run `bitbake build-sysroots`.
 #
 #  MACHINE            = Machine to run test against. The options are "witherspoon",
 #                       "palmetto", "romulus", or undefined (default).  Default