Set BRILLO_VENDOR_PARTITIONS to the list of files in the vendor directory.

builder will use BRILLO_VENDOR_PARTITIONS value to create a vendor partition
zip file, in folder out/dist.

Bug: 25692226
Change-Id: Ie135da2a897fd9d0edbeade2bbfd25033dc34949
Test: m dist, test with cl 183905
1 file changed