Set cameraserver optional.

Cameraserver is installed in handheld_system.mk, not
in TV and auto. Hence it is manually marked as optional.

Also update README to reflect what "optional" means.

Test: build cf_x86_auto
Bug: 37226359
Change-Id: Ib5e91c97256f9358074e68a67c535e25dec57409
2 files changed