ArmPkg/BdsLib: Fix uInitrd detection

ArmPkg/BdsLib: Fix uInitrd detection

This patch corrects a problem detecting uInitrd signature when booting
with FDT.

BdsBootLinuxFdt was attempting to read the signature from InitrdImage which
is zero at this point in the code.  The code now reads the signature from
InitrdImageBase.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14272 6f19259b-4bc3-4df7-8a09-765794883524