defaults.mk: default KEY_SIZE to 2048 in case of RSA algorithm

According to the documentation [1], KEY_SIZE defaults to 2048 when RSA
algorithm is chosen, so set this value on the make's defaults file.

[1] https://trustedfirmware-a.readthedocs.io/en/latest/getting_started/build-options.html



Change-Id: I030f98363198a752bc0dd03528f748de527d48d8
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>

make_helpers/defaults.mk

@@ -157,6 +157,11 @@ HW_ASSISTED_COHERENCY		:= 0
 # Set the default algorithm for the generation of Trusted Board Boot keys
 KEY_ALG				:= rsa
 
+# Set the default key size in case KEY_ALG is rsa
+ifeq ($(KEY_ALG),rsa)
+KEY_SIZE			:= 2048
+endif
+
 # Option to build TF with Measured Boot support
 MEASURED_BOOT			:= 0