build: create dependency file for device tree

If the DTS file includes other files, we need to depend on those to
ensure proper rebuild.

Change-Id: I0d319082aecd25146b36115a761be78b03e03fb1
build: mark .dtb section as writable, alloc

Marking the section as "alloc" declares that the section occupies
memory, and ensures that it is included in the final binary.
Previously, when the section became large enough, it overlapped the
.data section, and was not included in the binary.

I am unsure why it worked as a small section.

Change-Id: I8fff5c101a6c9e7b74def453a2a79dd7afb02219
armv7m: enable floating point extension at startup

Previously, the automatic floating point state preservation bit was set
in the floating point context control register.  This bit enables
floating point state preservation (automatic FP register saving on
exception entry) *only once the first FP instruction is executed*.

As a result, before the first FP instruction was executed, a task's
context would be saved without FP state.  If it was restored after an FP
instruction was executed, the core would "restore" the FP state, thus
corrupting the SP by "restoring" crucial parts of the task's stack.

Alternatively, set the FPCA in the CONTROL register at chip startup.
This enables FP state preservation from the start, and is the the bit
set by the core when the first FP instruction is executed when the ASPEN
bit is set.

Change-Id: I1e3456c4a150c52183e1a220fa74602f1ca88631