x86: use gc sections to reduce image size

Follow the discussion of Charles Manning and Mike Frysinger.
Using gc_sections helps reduce image size.

arch/i386/config.mk

@@ -26,3 +26,6 @@ CROSS_COMPILE ?= i386-linux-
 STANDALONE_LOAD_ADDR = 0x40000
 
 PLATFORM_CPPFLAGS += -DCONFIG_I386 -D__I386__
+
+LDFLAGS += --cref --gc-sections
+PLATFORM_RELFLAGS += -ffunction-sections

board/eNET/u-boot.lds

@@ -77,13 +77,13 @@ SECTIONS
 	_i386boot_bss_size = SIZEOF(.bss);
 
 	/* 16bit realmode trampoline code */
-	.realmode 0x7c0 : AT ( LOADADDR(.rel.dyn) + SIZEOF(.rel.dyn) ) { *(.realmode) }
+	.realmode 0x7c0 : AT ( LOADADDR(.rel.dyn) + SIZEOF(.rel.dyn) ) { KEEP(*(.realmode)) }
 
 	_i386boot_realmode = LOADADDR(.realmode);
 	_i386boot_realmode_size = SIZEOF(.realmode);
 
 	/* 16bit BIOS emulation code (just enough to boot Linux) */
-	.bios 0 : AT ( LOADADDR(.realmode) + SIZEOF(.realmode) ) { *(.bios) }
+	.bios 0 : AT ( LOADADDR(.realmode) + SIZEOF(.realmode) ) { KEEP(*(.bios)) }
 
 	_i386boot_bios = LOADADDR(.bios);
 	_i386boot_bios_size = SIZEOF(.bios);
@@ -98,12 +98,12 @@ SECTIONS
 	 * The fff0 offset of resetvec is important, however.
 	 */
 	. = 0xfffffe00;
-	.start32 : AT (TEXT_BASE + 0x3fe00) { *(.start32); }
+	.start32 : AT (TEXT_BASE + 0x3fe00) { KEEP(*(.start32)); }
 
 	. = 0xf800;
-	.start16 : AT (TEXT_BASE + 0x3f800) { *(.start16); }
+	.start16 : AT (TEXT_BASE + 0x3f800) { KEEP(*(.start16)); }
 
 	. = 0xfff0;
-	.resetvec : AT (TEXT_BASE + 0x3fff0) { *(.resetvec); }
+	.resetvec : AT (TEXT_BASE + 0x3fff0) { KEEP(*(.resetvec)); }
 	_i386boot_end = (LOADADDR(.resetvec) + SIZEOF(.resetvec) );
 }