Fix dependency issue for boot_ota_16k.zip

The dependency list and rule body of build rules are parsed
at different stages. This caused a problem, the dependency list is
parsed earlier, at which time INSTALLED_BOOTIMAGE_TARGET isn't
defined yet. But rule-body is parsed at a later time, when
INSTALLED_BOOTIMAGE_TARGET is already defined. This causes
non-deterministic build failures, as boot_ota_16k.zip uses
boot.img, but didn't list boot.img in dependency list. Build fails
if boot.img gets built after boot_ota_16k.zip. To mitigate:

1. Move boot_ota_16k after where INSTALLED_BOOTIMAGE_TARGET is defined
2. Use rule-local variables, this ensures that variables in rule-body
   are evaluated at the same stage as dependency list.

Test: m insatllclean; m boototapackage_16k
Bug: 397445008

Change-Id: Ia1f31d093700ea7152a533586bebe1d81f730b27