Let the bootloader know about ARMv7.

When an ARMv7 CPU is detected, immediately turn on the FPU. This allows
us to use vsnprintf in the TRACE call in that function, as our libc is
compiled with floating point support and will trigger a fault if the FPU
is not available.

This lets the boot go further, and crash in mmu_init. Next steps:
* Find why mmu_init is crashing
* Setup some fault handlers, otherwise we call uboot ones, and they are
not very helpful. They will also probably not work once the mmu is
enabledvery helpful. They will also probably not work once the mmu is
enabledvery helpful. They will also probably not work once the mmu is
enabled...