Allow clearer suppression of toolchain and compiler prefixes

Currently it is counterintuitive to disable toolchain prefixes: we need
to pass an empty space to it; passing an empty string would not disable
prefixes but instead instruct the build script to use a set of default
prefixes.

With this patch we restore the intuitive approach of passing an empty
string "" to disable the prefix.

Resolves COMPMID-5353, COMPMID-5380

Signed-off-by: SiCong Li <sicong.li@arm.com>
Change-Id: I2fe84689df9093cf6baf507dde44ca5ebf61023b
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/7686
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Gian Marco Iodice <gianmarco.iodice@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
diff --git a/docs/user_guide/release_version_and_change_log.dox b/docs/user_guide/release_version_and_change_log.dox
index 3632b27..ac6577b 100644
--- a/docs/user_guide/release_version_and_change_log.dox
+++ b/docs/user_guide/release_version_and_change_log.dox
@@ -41,6 +41,13 @@
 
 @section S2_2_changelog Changelog
 
+v22.08 Public major release
+ - Build flag change: toolchain_prefix, compiler_prefix:
+   - Use empty string "" to suppress any prefixes
+   - Use "auto" to use default (auto) prefixes chosen by the build script. This is the default behavior when unspecified
+   - Any other string will be used as custom prefixes to the compiler and the rest of toolchain tools
+   - The default behaviour when prefix is unspecified does not change, but its signifier has been changed from empty string "" to "auto"
+
 v22.05 Public major release
  - Various bug fixes.
  - Various optimizations.