sandbox: Allo sdl-config to be overridden

When cross-compiling, sometimes sdl-config must come from a different path
from the default. Add a way to override it, by adding SDL_CONFIG to the
environment before building U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/config.mk b/arch/sandbox/config.mk
index 31a12db..05fbbd7 100644
--- a/arch/sandbox/config.mk
+++ b/arch/sandbox/config.mk
@@ -5,14 +5,15 @@
 PLATFORM_CPPFLAGS += -DCONFIG_ARCH_MAP_SYSMEM
 PLATFORM_CPPFLAGS += -fPIC
 PLATFORM_LIBS += -lrt
+SDL_CONFIG ?= sdl-config
 
 # Define this to avoid linking with SDL, which requires SDL libraries
 # This can solve 'sdl-config: Command not found' errors
 ifneq ($(NO_SDL),)
 PLATFORM_CPPFLAGS += -DSANDBOX_NO_SDL
 else
-PLATFORM_LIBS += $(shell sdl-config --libs)
-PLATFORM_CPPFLAGS += $(shell sdl-config --cflags)
+PLATFORM_LIBS += $(shell $(SDL_CONFIG) --libs)
+PLATFORM_CPPFLAGS += $(shell $(SDL_CONFIG) --cflags)
 endif
 
 cmd_u-boot__ = $(CC) -o $@ -Wl,-T u-boot.lds $(u-boot-init) \
diff --git a/board/sandbox/README.sandbox b/board/sandbox/README.sandbox
index 48c1e2b..5e8fa58b 100644
--- a/board/sandbox/README.sandbox
+++ b/board/sandbox/README.sandbox
@@ -435,6 +435,13 @@
 This approach can be used on normal boards as well as sandbox.
 
 
+SDL_CONFIG
+----------
+
+If sdl-config is on a different path from the default, set the SDL_CONFIG
+environment variable to the correct pathname before building U-Boot.
+
+
 Testing
 -------