SPM: Makefile: Add targets to build resource description blobs

The targets cactus and ivy now also build the resource description blobs
used by Cactus and Ivy. This is done through a new shell script:
generate_dtb.sh

Example command line to build a package with Cactus and Ivy:

    CROSS_COMPILE=aarch64-linux-gnu- make PLAT=fvp   \
    DEBUG=1 LOG_LEVEL=50 TESTS=spm                   \
    tftf cactus ivy

    trusted_firmware/tools/sptool/sptool             \
        -o sp_package.bin                            \
        -i cactus.bin:cactus.dtb                     \
        -i ivy.bin:ivy.dtb

Also, add a disclaimer to explain that the current implementation of SPM
is a prototype that is going to undergo a lot of rework.

Change-Id: Iebc3ac28d20019b59f07f70f96fefc030f1a79ce
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/docs/user-guide.rst b/docs/user-guide.rst
index d070c6a..397b4c2 100644
--- a/docs/user-guide.rst
+++ b/docs/user-guide.rst
@@ -29,7 +29,7 @@
 
 ::
 
-    sudo apt-get install build-essential make git perl libxml-libxml-perl
+    sudo apt-get install device-tree-compiler build-essential make git perl libxml-libxml-perl
 
 Download and install the GNU cross-toolchain from Linaro. The TF-A Tests have
 been tested with version 6.2-2016.11 (gcc 6.2):