Android.bp: headers_install.sh: Use environment for unifdef tool

Tools used when generating headers_install.sh are run from within a
sandbox. The location of unifdef changes from headers_install.sh
generation time to usage time.

Instead, allow calling headers_install.py to specify the location of
unifdef with LOC_UNIFDEF environment variable. If this environment
variable is unset, unifdef is assumed to live in same directory as
generated headers_install.sh script. This assumption allows smooth
transition for users of headers_install.sh since main build is not
broken by this change.

Change-Id: Id23865dd3e25aa179e0bc6d3d6c549e0b93a6717
Signed-off-by: Elliot Berman <eberman@codeaurora.org>

Android.bp

@@ -11,12 +11,11 @@ genrule {
     srcs: ["scripts/headers_install.sh"],
     tools: ["unifdef"],
     out: ["headers_install.sh"],
-    cmd: "sed 's+scripts/unifdef+$(location unifdef)+g' $(in) > $(out)",
+    cmd: "sed 's+scripts/unifdef+$${LOC_UNIFDEF:-$$(dirname $$0)/unifdef}+g' $(in) > $(out)",
 }
 
-cc_prebuilt_binary {
+sh_binary_host {
     name: "headers_install.sh",
-    device_supported: false,
-    host_supported: true,
-    srcs: [":gen-headers_install.sh"],
+    src: ":gen-headers_install.sh",
+    required: ["unifdef"],
 }