support multiple kernels, boot.img output

In order to enable the delivery of multiple kernel variants with
a platform build, add support for generating boot.img files from
multiple kernels.

The BOARD_KERNEL_BINARIES macro may be defined as a list of kernel
binaries to be used (if not, the original behavior and assumption of a
single kernel binary is preserved). Each kernel binary filename
must begin with "kernel". Any extra text in the filename is appended
to the boot.img name, so "kernel-gz" would generate "boot-gz.img".

Bug: 151094943
Change-Id: Icbbf227e9561f4aa11ce10db8d7aa81c7a81eb5b
Merged-In: Icbbf227e9561f4aa11ce10db8d7aa81c7a81eb5b