Add support to build images for Raspberry Pi 2

[loos-br]

Add a new function to test the existence of u-boot ports.

Add a new board template for Raspberry Pi 2.

[brd]

Excellent, I will test this tonight!

[loos-br]

There are still some problems on FreeBSD side. You need build the kernel with 'options ARM_NEW_PMAP' and disable the build of framebuffer driver (or add a 'return (ENXIO)' early on bcm_fb_attach() (in bcm2835_fbd.c). I'm working on a fix for framebuffer.

[brd]

Why not add the ARM_NEW_PMAP to the RPI2 config in HEAD? If it is required, it seems to make sense to just have it there in the stock kernel.

[kientzle]

Nicely done!