Makefiles: Add / Update help for makefile targets

Currently running 'make help' just gives help for the kconfig targets.

This adds help for common coreboot and toolchain targets.  It stops
printing some of the less common kconfig targets, but still leaves
them in the makefile as documentation.

Change-Id: I2a00fcbc06f05dc4029a91f3dff830c19e4d1329
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/12458
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

Makefile.inc

@@ -446,6 +446,17 @@ gitconfig:
 	git config remote.origin.push HEAD:refs/for/master
 	(git config --global user.name >/dev/null && git config --global user.email >/dev/null) || (printf 'Please configure your name and email in git:\n\n git config --global user.name "Your Name Comes Here"\n git config --global user.email your.email@example.com\n'; exit 1)
 
+help::
+	@echo  '*** Toolchain ***'
+	@echo  '  crossgcc        - Build coreboot cross-compilers for all platforms'
+	@echo  '  crosstools      - Build coreboot cross-compiler and GDB for all platforms'
+	@echo  '  crossgcc-clean  - Remove all built coreboot cross-compilers'
+	@echo  '  crossgcc-ARCH   - Build cross-compiler for specific architecture'
+	@echo  '  crosstools-ARCH - Build cross-compiler with GDB for specific architecture'
+	@echo  '  ARCH can be "i386", "x64", "arm", "aarch64", "mips", or "riscv"'
+	@echo  '  Use "make [target] CPUS=#" to build toolchain using multiple cores'
+	@echo
+
 # For the toolchain builds, use CPUS=x to use multiple processors to build
 # use BUILDGCC_OPTIONS= to set any crossgcc command line options
 # Example: BUILDGCC_OPTIONS='-t' will keep temporary files after build