plat: Fix build issue for qemu and rpi3 platforms

Coverity build periodically throws below errors(non-consistently)
for 'QEMU' and 'RPI3' platforms.

/bin/sh: 1: cannot create build/qemu/debug/rot_key.pem: Directory
nonexistent
plat/qemu/qemu/platform.mk:86: recipe for target 'build/qemu/debug/
rot_key.pem' failed
make: *** [build/qemu/debug/rot_key.pem] Error 2

/bin/sh: 1: cannot create /work/workspace/workspace/tf-coverity/build
/rpi3/debug/rot_key.pem: Directory nonexistent
plat/rpi/rpi3/platform.mk:214: recipe for target '/work/workspace/
workspace/tf-coverity/build/rpi3/debug/rot_key.pem' failed
make: *** [/work/workspace/workspace/tf-coverity/build/rpi3/debug/
rot_key.pem] Error 2

Issue seems to be occurred when 'ROT key' is generated before creating
the platform build folder(for e.g.build/qemu/debug).

Changes are made to fix this issue by adding orderly dependancy of
the platform folder for the 'ROT key' creation which ensures that
platform folder is created before generating 'ROT key'.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I20c82172dde84e4c7f2373c0bd095d353f845d38