bootimg bbclass: don't hardcode 'bzImage' name

In the EFI case there are 2 more architectures besides x86(_64) that may use this class: aarch32, which tends to use 'zImage' and aarch64, which uses 'Image'. Instead of hardcoding those values like the luvOS fork does, use the proper variable.

Signed-off-by: Koen Kooi <koen.kooi@linaro.org>
Signed-off-by: Ross Burton <ross.burton@intel.com>