[DRIVERS] Allow using i.MX6 UART without prior bootloader initialization.

This patch configures and enables the UART FIFO, ignore the RTS, and set the
UART mode to 8N1.

Signed-off-by: Jimmy Durand Wesolowski <jimmy.durand-wesolowski@openwide.fr>
[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.

Signed-off-by: Jimmy Durand Wesolowski <jimmy.durand-wesolowski@openwide.fr>
[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.

Signed-off-by: Jimmy Durand Wesolowski <jimmy.durand-wesolowski@openwide.fr>