build-buildroot.sh: restrict packing IOMMU artifacts

If "$LINUX_OUTDIR/tools/iommu/smmute" folder exists on the host
machine, IOMMU artifacts would be packed to be part of the ext3
partition targetted for grub ramdisk. This works fine in general cases.
However, if LINUX_TOOLS_IOMMU_BUILD is set to 0 and the previous build
is not cleaned, the scripts end up with failed access on empty
"$LINUX_OUTDIR/tools/iommu/smmute" directory. Prevent this issue from
happening by adding an additional check to see if
LINUX_TOOLS_IOMMU_BUILD is set to 1 before proceeding with adding IOMMU
artifacts.

Signed-off-by: Rohit Mathew <rohit.mathew@arm.com>
Change-Id: Ia5da464c715028e478e298925447f15f4e9bd9ff