Add an option to select whether the shared library is built

The shared library is useful only with sandbox, so add an option to
allow it to be disabled, e.g. for EFI builds.

Signed-off-by: Simon Glass <simon.glass@canonical.com>

Kconfig

@@ -106,6 +106,16 @@ config ULIB
 
 	  The library is called libu-boot.so
 
+config ULIB_SHARED_LIB
+	bool "Build shared library (libu-boot.so)"
+	depends on ULIB && SANDBOX
+	default y
+	help
+	  Enable this to build the shared library version (libu-boot.so) in
+	  addition to the static library (libu-boot.a). Disable this if you
+	  only need the static library, which can speed up builds and avoid
+	  linking issues with some configurations.
+
 config OPTIMIZE_INLINING
 	bool "Allow compiler to uninline functions marked 'inline' in full U-Boot"
 	help