Strip whitespace from INTERNAL_VENDOR_RAMDISK_FRAGMENTS

aosp/1664081 introduces a bug where if BOARD_VENDOR_RAMDISK_FRAGMENTS is
empty, then INTERNAL_VENDOR_RAMDISK_FRAGMENTS would become " " (single
whitespace).
Just unconditionally $(strip ...) the variable to remove any extra
whitespace.

Bug: 183395459
Test: m dist and check *-target_files-*.zip
Change-Id: Ic842756f3a64c073593592d22c980820664e11c1