toolchain/external/custom: add choice to select 'AT_LEAST_XXX' header version

Select the appropriate BR2_TOOLCHAIN_HEADERS_AT_LEAST_XXX options for the
external, custom toolchain backend.

We try to be conservative here, and default to kernel headers 2.6.x.

[Thomas: remove duplicated depends on BR2_TOOLCHAIN_EXTERNAL_CUSTOM,
since the choice is already inside a if BR2_TOOLCHAIN_EXTERNAL_CUSTOM
... endif block.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

toolchain/toolchain-external/Config.in

@@ -944,6 +944,84 @@ config BR2_TOOLCHAIN_EXTERNAL_MUSL
 
 if BR2_TOOLCHAIN_EXTERNAL_CUSTOM
 
+choice
+	bool "External toolchain kernel headers series"
+	default BR2_TOOLCHAIN_EXTERNAL_HEADERS_REALLY_OLD
+	help
+	  Set to the kernel headers version that were used to build
+	  this external toolchain.
+
+	  This is used to hide/show some packages that have strict
+	  requirements on the version of kernel headers.
+
+	  If unsure what version your toolchain is using, you can look
+	  at the value of LINUX_VERSION_CODE in linux/version.h in your
+	  toolchain. The Linux version is M.m.p, with:
+	    M = ( LINUX_VERSION_CODE >> 16 ) & 0xFF
+	    m = ( LINUX_VERSION_CODE >> 8  ) & 0xFF
+	    p = ( LINUX_VERSION_CODE >> 0  ) & 0xFF
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_13
+	bool "3.13.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_13
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_12
+	bool "3.12.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_12
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_11
+	bool "3.11.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_11
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_10
+	bool "3.10.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_10
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_9
+	bool "3.9.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_9
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_8
+	bool "3.8.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_8
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_7
+	bool "3.7.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_7
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_6
+	bool "3.6.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_6
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_5
+	bool "3.5.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_5
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_4
+	bool "3.4.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_4
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_3
+	bool "3.3.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_3
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_2
+	bool "3.2.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_2
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_1
+	bool "3.1.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_1
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_0
+	bool "3.0.x"
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_0
+
+config BR2_TOOLCHAIN_EXTERNAL_HEADERS_REALLY_OLD
+	bool "2.6.x"
+
+endchoice
+
 choice
 	prompt "External toolchain C library"
 	default BR2_TOOLCHAIN_EXTERNAL_CUSTOM_UCLIBC