Add Juno AArch32 and AArch64 User Guide instructions

Updated section 6, building a FIP for Juno and FVP, adding
instructions for AArch32 and AArch64.

Updated section 4.1, summary of build options, to include a
description of the `JUNO_AARCH32_EL3_RUNTIME` build flag.

Change-Id: I4ed006522cab981371c382859063f088fbfcb8f7
Signed-off-by: Isla Mitchell <isla.mitchell@arm.com>