gem5: centralise v20 sources

Both simulator and bootloader now fetch from a single include file
defining the sources. This is in preparation for m5term recipe.

Signed-off-by: Adrian Herrera <adrian.herrera@arm.com>
Change-Id: Ia4f6b6d99401ddb0c726d913ceff4ced90f41f6b
Signed-off-by: Jon Mason <jon.mason@arm.com>