stand: Move eficom to libefi

Rename efiserialc to eficom.c and move it to libefi. Remove
loader.efi.h, since it's not needed. It's architecture independent
(though how we use it might vary). Drivers also belong in libfoo
in the boot loader: all the BIOS drivers are in i386/libi386 and
the console driver is in efi/libefi.

Sponsored by:		Netflix
Reviewed by:		tsoome
Differential Revision:	https://reviews.freebsd.org/D40007

stand/efi/libefi/Makefile

@@ -11,6 +11,7 @@ SRCS=	delay.c \
 	efi_console.c \
 	efi_driver_utils.c \
 	efichar.c \
+	eficom.c \
 	efienv.c \
 	efihttp.c \
 	efinet.c \

stand/efi/libefi/eficom.c

@@ -34,8 +34,6 @@ __FBSDID("$FreeBSD$");
 #include <efi.h>
 #include <efilib.h>
 
-#include "loader_efi.h"
-
 static EFI_GUID serial = SERIAL_IO_PROTOCOL;
 
 #define	COMC_TXWAIT	0x40000		/* transmit timeout */

stand/efi/loader/arch/amd64/Makefile.inc

@@ -5,7 +5,6 @@ SRCS+=	amd64_tramp.S \
 	elf64_freebsd.c \
 	trap.c \
 	multiboot2.c \
-	efiserialio.c \
 	exc.S
 
 .PATH:	${BOOTSRC}/i386/libi386

stand/efi/loader/arch/arm/Makefile.inc

@@ -1,7 +1,6 @@
 # $FreeBSD$
 
 SRCS+=	exec.c \
-	efiserialio.c \
 	start.S
 
 HAVE_FDT=yes

stand/efi/loader/arch/arm64/Makefile.inc

@@ -3,7 +3,6 @@
 HAVE_FDT=yes
 
 SRCS+=	exec.c \
-	efiserialio.c \
 	start.S
 
 .PATH:	${BOOTSRC}/arm64/libarm64

stand/efi/loader/arch/riscv/Makefile.inc

@@ -3,5 +3,4 @@
 HAVE_FDT=yes
 
 SRCS+=	exec.c \
-	efiserialio.c \
 	start.S