util/crossgcc: Add option to get packages from coreboot's mirror

coreboot has been keeping a mirror of all the toolchain packages used
for releases for quite a while now.  This adds an option to fetch the
packages from the coreboot mirror directly to buildgcc.

This can help with both our releases and when one of the various
servers experiences interruptions or changes a path.

To do this, the URL and filename needed to be split apart, which led to
quite a few changes in the buildgcc script.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I7df58dca152e7bfe9fde34d290e05b52515b20d9
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70053
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Felix Singer <felixsinger@posteo.net>

util/crossgcc/Makefile.inc

@@ -3,6 +3,7 @@
 TOOLCHAIN_ARCHES := i386 x64 arm aarch64 riscv ppc64 nds32le
 
 help_toolchain help::
+	@echo
 	@echo  '*** Toolchain targets ***'
 	@echo  '  crossgcc        - Build coreboot cross-compilers for all platforms'
 	@echo  '  crossgcc-clean  - Remove all built coreboot cross-compilers'
@@ -14,6 +15,7 @@ help_toolchain help::
 	@echo  '  ARCH can be "$(subst $(spc),"$(comma) ",$(TOOLCHAIN_ARCHES))"'
 	@echo  '  Use "make [target] CPUS=#" to build toolchain using multiple cores'
 	@echo  '  Use "make [target] DEST=some/path" to install toolchain there'
+	@echo  '  Use "make [target] BUILDGCC_OPTIONS="-m" to get packages from coreboot mirror"
 	@echo
 
 # For the toolchain builds, use CPUS=x to use multiple processors to build