tools/wltests: split flash images in two steps

To properly flash some devices, it can be required to do some operations
in ADB mode before actually rebooting into the bootloader. This allows
for example to push new kernel modules on the target before flashing a
new kernel image from FASTBOOT mode.

This can be achieved by introducing the support for flash_images to be
called from two different contexts:
1) ADB mode: with the currently running kernel, so that we can push
     the modules for a new kernel
2) FASTBOOT mode: when the device has been rebooted into bootloader, and
     thus we can safely update the kernel image since the modules have
     been already pushed

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>