sandbox: allow cross-compiling sandbox

UEFI test files like helloworld.efi require an architecture specific
PE-COFF header.

Currently this does not work for cross compiling. If $CROSS_COMPILE is set,
use the first part of the architecture triplet from the variable to
choose the PE-COFF header.

Now we can cross-compile the sandbox, e.g.

    make sandbox_defconfig NO_SDL=1
    CROSS_COMPILE=/opt/bin/aarch64-linux-gnu- NO_SDL=1 MK_ARCH=aarch64 make
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: NSimon Glass <sjg@chromium.org>

Makefile

@@ -17,9 +17,13 @@ NAME =
 # o Look for make include files relative to root of kernel src
 MAKEFLAGS += -rR --include-dir=$(CURDIR)
 
-# Determine host architecture
+# Determine target architecture for the sandbox
 include include/host_arch.h
-MK_ARCH="${shell uname -m}"
+ifeq ("", "$(CROSS_COMPILE)")
+  MK_ARCH="${shell uname -m}"
+else
+  MK_ARCH="${shell echo $(CROSS_COMPILE) | sed -n 's/^\s*\([^\/]*\/\)*\([^-]*\)-\S*/\2/p'}"
+endif
 unexport HOST_ARCH
 ifeq ("x86_64", $(MK_ARCH))
   export HOST_ARCH=$(HOST_ARCH_X86_64)
@@ -27,7 +31,7 @@ else ifneq (,$(findstring $(MK_ARCH), "i386" "i486" "i586" "i686"))
   export HOST_ARCH=$(HOST_ARCH_X86)
 else ifneq (,$(findstring $(MK_ARCH), "aarch64" "armv8l"))
   export HOST_ARCH=$(HOST_ARCH_AARCH64)
-else ifeq ("armv7l", $(MK_ARCH))
+else ifneq (,$(findstring $(MK_ARCH), "arm" "armv7" "armv7l"))
   export HOST_ARCH=$(HOST_ARCH_ARM)
 else ifeq ("riscv32", $(MK_ARCH))
   export HOST_ARCH=$(HOST_ARCH_RISCV32)