aemfvp-a: Add changes to support busybox and distro boot on Armv-A Base AEM FVP

Add changes in the existing build scripts to support busybox and
distribution boot of Armv-A Base AEM FVP Platform.
The following boot methods are verified by using these changes,
1. AARCH64 :  UEFI + busybox
2. AARCH64 :  U-Boot + busybox
3. AARCH64 :  Distribution (Ubuntu, Fedora, and Debian) boot

And the following changes are also included in this commit.

1. Remove the dependency on uefi-tools. These changes are merged
   from 'refinfra/fb1917c0'.
2. Use the latest GNU v10.3 toolchain from developer.arm.com.
3. Add build-test-busybox.sh for base FVP to create the disk
   image for the busybox boot.
4. Add the fvp platform name in build-test-uefi.sh for distribution
   boot support.

Signed-off-by: Selvarasu Ganesan <selvarasu.ganesan@arm.com>
Change-Id: I9b2c3aa130f21f1fbf1e7f5cd8474dd53ecd0f8a