Add vendor ramdisk name to vendor_boot v4

Add a ramdisk_name field to struct vendor_ramdisk_table_entry_v4 to
uniquely identify each vendor ramdisk. The ramdisk_name field is a ascii
text field, length 32. The ramdisk_name must be unique.

Bug: 177882072
Test: atest mkbootimg_test --host
Change-Id: I63d3d755d44bb7fa6ff6c947466532abcc9e9c47