toolchain: Select USE_SSTRIP with external musl toolchain

When we use the internal toolchain USE_SSTRIP will be selected by
default for musl libc and USE_STRIP when glibc is used. Do the same when
an external toolchain is used. USE_GLIBC will also be set for external
toolchain builds based on the EXTERNAL_TOOLCHAIN_LIBC_USE_GLIBC setting.

Signed-off-by: Hauke Mehrtens <[email protected]>
(cherry picked from commit 9403810c020cca136149973a3929bf77a1f501aa)

config/Config-build.in

@@ -164,7 +164,6 @@ menu "Global build settings"
 
 	choice
 		prompt "Binary stripping method"
-		default USE_STRIP   if EXTERNAL_TOOLCHAIN
 		default USE_STRIP   if USE_GLIBC
 		default USE_SSTRIP
 		help

target/sdk/files/Config.in

@@ -31,7 +31,6 @@ menu "Global build settings"
 
 	choice
 		prompt "Binary stripping method"
-		default USE_STRIP   if EXTERNAL_TOOLCHAIN
 		default USE_STRIP   if USE_GLIBC
 		default USE_SSTRIP
 		help