[CORE] Avoid the "always_inline function might not be inlinable" warning.

GCC displays a warning about the fact a function may not be inlined even if
told so, if no optimization is asked for. To avoid this, as explained in the
GCC manual :
Generally, functions are not inlined unless optimization is specified. For
functions declared inline, this attribute inlines the function even if no
optimization level is specified.

Adding "inline" correct this warning.
[MAKEFILE] Override the V environment variable.

If the Xvisor makefile is called by another script or makefile defining V, it
breaks the verbosity option of the build system, concatenating its value in
front of each command.
[MAKEFILE] Use a CC preprocessor on device tree files.

Using a CC preprocessor on device tree files allows to add C header include
directives in device tree files.
Also, the device tree compiler is now set as a dependency of the dependency
file generation as it is used to create them.
[ARM] Correcting licenses.

The old license remained, set as default until the final one was approved.
[DRIVERS] i.MX6Q UART TX interrupt corrected and menuconfig option added.

If the menuconfig option is set, the TX interrupt is enabled, allowing to
sleep when the TX FIFO is full, and to be awaken when it has been cleared.
[DRIVERS] MTD integration started.
[ARM] Adding the i.MX6 SPI driver in the device tree.
[DRIVERS] Starting the SPI driver port.
arm32: Fix strex inline asm compiling error.

Signed-off-by: CoryXie <cory.xie@gmail.com>
[MAKEFILE] Simplify dependency file generation.

Echoing the directory of the dependency target and then running '-MM' is not
optimized. Using '-MT' allows to specify the dependency target name.
[DRIVERS] Adding MXC GPIO driver.

This driver is required by the i.MX SPI driver.
For now, I tried not to use the generic GPIO driver, as it would increase code
size without being necessary.
This driver cannot be tested yet.