Change the type of the "default" vendor ramdisk to VENDOR_RAMDISK_TYPE_PLATFORM

Even though the "default" ramdisk now contains platform + recovery +
vendor DLKMs, giving it the PLATFORM type is still more suitable than
the NONE type, as the "default" ramdisk should always be loaded during
boot. In other words, since it contains platform bits, it must be always
loaded, thus PLATFORM type.
Also polish the comments.

Bug: 162864255
Test: Presubmit
Test: atest --host mkbootimg_test
Test: atest --host fastboot_vendor_boot_img_utils_test
Change-Id: Ibed408df695e28209affd72d20ea8477a8db00cc