rmodule: use program.ld for linking

Bring rmodule linking into the common linking method.
The __rmodule_entry symbol was removed while using
a more common _start symbol. The rmodtool will honor
the entry point found within the ELF header. Add
ENV_RMODULE so that one can distinguish the environment
when generating linker scripts for rmodules. Lastly,
directly use program.ld for the rmodule.ld linker script.

BUG=chrome-os-partner:44827
BRANCH=None
TEST=Built rambi and analyzed the relocatable ramstage,
     sipi_vector, and smm rmodules.

Change-Id: Iaa499eb229d8171272add9ee6d27cff75e7534ac
Signed-off-by: Aaron Durbin <adubin@chromium.org>
Reviewed-on: http://review.coreboot.org/11517
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/cpu/x86/sipi_vector.S

@@ -56,10 +56,8 @@ ap_count:
 
 .text
 .code16
-.global ap_start
-.global __rmodule_entry
-__rmodule_entry:
-ap_start:
+.global _start
+_start:
 	cli
 	xorl	%eax, %eax
 	movl	%eax, %cr3    /* Invalidate TLB*/
@@ -74,9 +72,9 @@ ap_start:
 
 	/* The gdtaddr needs to be releative to the data segment in order
 	 * to properly dereference it. The .text section comes first in an
-	 * rmodule so ap_start can be used as a proxy for the load address. */
+	 * rmodule so _start can be used as a proxy for the load address. */
 	movl	$(gdtaddr), %ebx
-	sub	$(ap_start), %ebx
+	sub	$(_start), %ebx
 
 	data32 lgdt (%ebx)