scripts/mkbootimg-zee.sh: use prebuilt dtatt.img

Signed-off-by: Hieu Phan <aznrice2k4@gmail.com>
Makefile: change opt flag from cortex-a15 to cortex-a15.cortex-a7

Signed-off-by: Hieu Phan <aznrice2k4@gmail.com>
Makefile: use generic-armv7-a

Signed-off-by: Hieu Phan <aznrice2k4@gmail.com>