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/arch/x86/c_start.S

@@ -23,8 +23,6 @@ thread_stacks:
 	.code32
 #endif
 	.globl _start
-	.globl __rmodule_entry
-__rmodule_entry:
 _start:
 	cli
 	lgdt	%cs:gdtaddr

src/arch/x86/include/arch/header.ld

@@ -24,7 +24,7 @@ PHDRS
 	to_load PT_LOAD;
 }
 
-#if ENV_RAMSTAGE
+#if ENV_RAMSTAGE || ENV_RMODULE
 ENTRY(_start)
 #elif ENV_ROMSTAGE
 ENTRY(protected_start)