Add SYSROOT, EXTRA_{LDFLAGS,LDLIBS} config opts; sample Android NDK config

Signed-off-by: Rob Walker <rwalker@rwalker.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/Makefile.flags b/Makefile.flags
index ee4c518..68dfa57 100644
--- a/Makefile.flags
+++ b/Makefile.flags
@@ -104,6 +104,11 @@
 LDLIBS += m
 endif
 
+ifneq ($(CONFIG_SYSROOT),)
+CFLAGS += --sysroot=$(CONFIG_SYSROOT)
+export SYSROOT=$(CONFIG_SYSROOT)
+endif
+
 ifeq ($(CONFIG_PAM),y)
 # libpam uses libpthread, so for static builds busybox must be linked to
 # libpthread. On some platforms that requires an explicit -lpthread, so
@@ -137,6 +142,16 @@
 SKIP_STRIP = y
 endif
 
+ifneq ($(CONFIG_EXTRA_LDFLAGS),)
+EXTRA_LDFLAGS += $(strip $(subst ",,$(CONFIG_EXTRA_LDFLAGS)))
+#"))
+endif
+
+ifneq ($(CONFIG_EXTRA_LDLIBS),)
+LDLIBS += $(strip $(subst ",,$(CONFIG_EXTRA_LDLIBS)))
+#"))
+endif
+
 # Busybox is a stack-fatty so make sure we increase default size
 # TODO: use "make stksizes" to find & fix big stack users
 # (we stole scripts/checkstack.pl from the kernel... thanks guys!)