build(juno): added error check for BL32 dependency

Macro PLAT_ARM_MAX_BL32_SIZE definition is dependent on
JUNO_AARCH32_EL3_RUNTIME=1. When this value is not set and building
for AArch32, the build fails as it cannot find the definition of the
first macro. With this patch, the problem is addressed by producing
an error when the dependency is not set properly.

Change-Id: Ibe4e976bf79892fd26f3b266bd546218f5616825
Signed-off-by: Juan Pablo Conde <juanpablo.conde@arm.com>