misc: Reflect changes of arm bootloader name

misc: Reflect changes of arm bootloader name

With https://gem5-review.googlesource.com/c/public/gem5/+/22687 the
VExpress_GEM5_Base platform is changing the required bootloader name
by removing the _emm suffix.
While this had been changed in the prebuilt binaries in gem5.org, it
hadn't in the bootloader makefiles or in other utility functions.

The patch is not completely removing the _emm bootloaders since those
are still used by VExpress_EMM and VExpress_EMM64 platforms.

Change-Id: Iea3148eab313ab06cf2e74660e11708e1a22ce5f
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Adrian Herrera <adrian.herrera@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23947
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>