Always use generated boot.img for `fastboot boot` command

I've added the -c parameter to the fastboot boot call which allows to override the kernel cmdline, in pmbootstrap was already handled to use the cmdline from the deviceinfo or the one supplied with the pmbootstrap flasher boot --cmdline parameter.

I've tested it with the HTC Desire, please test it with other fastboot compatible devices.

This change is necessary for the new devices that have the DTBs attached to the boot.img, since we were using the fastboot boot command with all the parts provided separated and there isn't the possibility to specify the DTB as a parameter