arch: BINFMT_FLAT_SHARED is not really shared for buildroot purposes

Although BINFMT_FLAT_SHARED is indeed a shared library format, it does
not support dynamic library loading with dlopen(). So for buildroot
purposes, BR2_STATIC_LIBS shouldn't be selected.

As it happens, the compiler options that are added for
BINFMT_FLAT_SHARED also make the compiler ignore the -static option, so
we can simply force BR2_STATIC_LIBS and things work out perfectly.

Therefore, remove the select of BR2_BINFMT_SUPPORTS_SHARED from
BINFMT_FLAT_SHARED, which in turn makes sure that BR2_STATIC_LIBS is
selected.

[Arnout: rewrite commit message, add explanatory comment]

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

arch/Config.in

@@ -295,7 +295,12 @@ config BR2_BINFMT_FLAT_SEP_DATA
 
 config BR2_BINFMT_FLAT_SHARED
 	bool "Shared binary"
-	select BR2_BINFMT_SUPPORTS_SHARED
+	# Even though this really generates shared binaries, there is no libdl
+	# and dlopen() cannot be used. So packages that require shared
+	# libraries cannot be built. Therefore, we don't select
+	# BR2_BINFMT_SUPPORTS_SHARED and therefore force BR2_STATIC_LIBS.
+	# Although this adds -static to the compilation, that's not a problem
+	# because the -mid-shared-library option overrides it.
 	help
 	  Allow to load and link indiviual FLAT binaries at run time.