Fix incrementality issues for vendor_boot_filesystem_config.txt

Similar to the issue described in https://r.android.com/3524966,
vendor_boot_filesystem_config.txt runs into incrementality issues since
the fs_config call is in a separate rule from the rule which assembles
VENDOR_BOOT/RAMDISK.

This CL fixes this by inlining the fs_config cmd in the rule that builds
target_files.zip

Test: In a clean build, diffed vendor_boot_filesystem_config.txt between
make and soong

Change-Id: Ide86b81b7b97316eb2dee56b0e3658ad84b1bc6e