flashing wrong images
This is quite likely you are missing to flash boot.img file, you could use below steps to build boot.img for UEFI compatible:
#!/bin/sh export ARCH=arm64 export CROSS_COMPILE=aarch64-linux-gnu- BOOT=arch/arm64/boot DTB=hi3660-hikey960.dtb make hikey960_defconfig make Image dtbs -j8 RAMDISK=/home/leoy/work3/hikey960/aosp_upstream/out/target/product/hikey960/ramdisk.img cat $BOOT/Image $BOOT/dts/hisilicon/$DTB > $BOOT/Image-dtb abootimg --create $BOOT/boot.img -k $BOOT/Image-dtb -r $RAMDISK -f bootimg-960.cfg
Below is bootimg-960.cfg for the reference:
bootsize = 0x1500000 pagesize = 0x800 kerneladdr = 0x80000 ramdiskaddr = 0x7c00000 secondaddr = 0xf00000 tagsaddr = 0x7a00000 name = cmdline = loglevel=15 androidboot.hardware=hikey960 androidboot.selinux=permissive firmware_class.path=/system/etc/firmware
For more complete steps, you also could refer this script for building boot.img for UEFI compatible:
Unfortunately there have no official boot.img can share with you, there have one: https://builds.96boards.org/snapshots/hikey960/linaro/aosp-master/latest/boot.uefi.img, but I can see this image we cannot download with permission issue.
Just in case you want to do quick try, you could download my prebuilt boot.img:
http://people.linaro.org/~leo.yan/hikey960/boot.img; if after you flash this image “sudo fastboot flash boot boot.img” and can dismiss “Synchronous Exception”, that means you can just focus on building boot.img with upper sharing method.
I will try with your prebuilt images and let you know.