Some kernel packages (e.g. linux-lts in Alpine) don't ship linux.efi, so
this needs to fallback to "vmlinuz" or else it won't be able to find a
kernel for boot-deploy.
As we move towards UEFI on more devices, we want to use systemd-boot and
kernel images built with CONFIG_ZBOOT. However, these images aren't
compatible with existing Android bootloaders. As a result, we must
install both the old vmlinuz image for old bootloaders, and the fancy
new self-extracting EFI image.
When using systemd_boot, use linux.efi as the kernel file name instead
of globbing on vmlinuz*.
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
Checking for the app is unnecessary, since the cmd.Run later will fail
if it doesn't exist. This allows dropping the dependency on misc.Exists.
There's also no reason to print that the command failed, just return the
error.
