Store additional information in image header.

Store the reservation needed for loading the boot image and
the number of boot class path components. The former helps
avoid one extra file open+read+close when loading the boot
image and the latter shall be used in subsequent CLs to load
boot image for a partial boot class path.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: Pixel 2 XL boots.
Bug: 119868597
Change-Id: I01dba923cfa3f8faf0e41a4139b8913c78404d54