toolchain-external-custom: Describe BR2_TOOLCHAIN_EXTERNAL_CUSTOM_PREFIX

"Toolchain prefix" option apparently doesn't have any help describing
it, which causes confusion when using external toolchain. Leaving this
option at default prefix name ("$(ARCH)-linux") when external toolchain
components are called with different prefix (e.g.
"$(ARCH)-unknown-linux-gnu") may cause build failure unless the prefix
symlink is already in place (e.g. when using Buildroot-generated
toolchain as external toolchain).

Describe the option to clarify.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit e418a2c06a1bdc7a5fc65ce58400ed0a9c9e28bb)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

toolchain/toolchain-external/toolchain-external-custom/Config.in.options

@@ -24,6 +24,21 @@ config BR2_TOOLCHAIN_EXTERNAL_REL_BIN_PATH
 config BR2_TOOLCHAIN_EXTERNAL_CUSTOM_PREFIX
 	string "Toolchain prefix"
 	default "$(ARCH)-linux"
+	help
+	  Component name prefix used by your external toolchain.
+
+	  Typically, this is the target tuple, e.g. something that
+	  looks like: $(ARCH)-unknown-linux-gnu.
+
+	  If unsure, look at the "bin" directory of your toolchain and
+	  note the part of component name before the last dash, i.e. if
+	  you call the toolchain's gcc as $(ARCH)-unknown-linux-gnu-gcc,
+	  the prefix is $(ARCH)-unknown-linux-gnu.
+
+	  Note that Buildroot toolchains have shorter aliases that look
+	  like $(ARCH)-linux, as symlinks to the full-tuple components,
+	  so you can leave this option at default value if you use them
+	  as external toolchain.
 
 choice
 	bool "External toolchain gcc version"