Fix #6294: Full HW Random Generator

Change-Id: I9babada92991c646d844a25af175150d530a8ddb
Reviewed-on: https://gerrit.st.com/11597
Reviewed-by: Jean-Michel DELORME <jean-michel.delorme@st.com>
Tested-by: Jean-Michel DELORME <jean-michel.delorme@st.com>
Reviewed-by: Jocelyn RICARD <jocelyn.ricard@st.com>
Reviewed-by: Pascal BRAND <pascal.brand@st.com>

Conflicts:
	core/arch/st231/plat-stm/rng_support.c
Fix #6323: A failing RSA decode leads to panic

Change-Id: Ia4762f076922338f280d431f104b653e731bf64f
Reviewed-on: https://gerrit.st.com/11598
Reviewed-by: Jean-Michel DELORME <jean-michel.delorme@st.com>
Tested-by: Jean-Michel DELORME <jean-michel.delorme@st.com>
Tested-by: Jocelyn RICARD <jocelyn.ricard@st.com>
Reviewed-by: Pascal BRAND <pascal.brand@st.com>

Update wrt comments from review

Fix returned error when the mode is not ok
Fix TA build error

This fixes a bug introduced by commit 4334e8d. In ta_dev_kit.mk, out-dir must
be set to '.' and not left empty when output directory (O=) is not specified.
Otherwise, when building a trusted app, the compile rule will have an unwanted
leading slash (result from the expansion of $(out-dir)/$(...) in compile.mk).
Error is:

make[1]: Entering directory `/home/jerome/tmp/lcu14_optee_hello_world/ta'
  CC      /hello_world_ta.o
hello_world_ta.c:101:1: fatal error: opening dependency file /.hello_world_ta.o.d: Permission denied
 }
 ^
compilation terminated.