scripts: Makefile.lib: generate dsdt_generated.c instead of dsdt.c

There is a conflict between the static file
lib/acpi/dsdt.c and the file dsdt.c generated
dynamicaly by scripts/Makefile.lib. When a
mrproper is done, the static file dsdt.c is
removed. If a build with acpi enabled is
launched after, the following error is raised:

  CC      lib/acpi/acpi_table.o
make[2]: *** No rule to make target 'lib/acpi/dsdt.asl', needed by 'lib/acpi/dsdt.c'.  Stop.
scripts/Makefile.build:394: recipe for target 'lib/acpi' failed

To avoid such error, the generated file is named
dsdt_generated.c instead of dstdt.c.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Tested-by: Heiko Thiery <heiko.thiery@gmail.com>