16kb: Remove PRODUCT_BOOTS_16K build variable

The PRODUCT_BOOTS_16K is removed and will be
replaced by TARGET_BOOTS_16K variable.

The TARGET_BOOTS_16K will drive the kernel and
modules to use instead.

Bug: 350529323
Bug: 346868433
Test: Built and boot pgagnostic targets
Flag: EXEMPT bugfix
Change-Id: If539f8a9f8c87650b8f618f302eee2c284d8ec79