kboot: Move _start out of kboot and into libkboot

Move the startup code from kboot/kboot to kboot/libkboot and add the
necessary infrastructure for it to build. move start_arch.h, a private
header for libkboot, over as well.

Sponsored by:		Netflix

(cherry picked from commit a03411e84728e9b267056fd31c7d1d9d1dc1b01e)

stand/kboot/Makefile

@@ -2,6 +2,6 @@ NO_OBJ=t
 
 .include <bsd.init.mk>
 
-SUBDIR+=	kboot
+SUBDIR+=	libkboot .WAIT kboot
 
 .include <bsd.subdir.mk>

stand/kboot/Makefile.inc

@@ -1 +1,7 @@
+# Bring in the mini-libc that we wrote
+
+KBOOTSRC=${BOOTSRC}/kboot
+CFLAGS+=-I${KBOOTSRC}/include
+LIBKBOOT=${BOOTOBJ}/kboot/libkboot/libkboot.a
+
 .include "../Makefile.inc"

stand/kboot/kboot/Makefile

@@ -21,7 +21,6 @@ INSTALLFLAGS=	-b
 SRCS=	\
 		bootinfo.c \
 		conf.c \
-		crt1.c \
 		gfx_fb_stub.c \
 		host_syscalls.c \
 		hostcons.c \
@@ -63,7 +62,7 @@ CFLAGS+=	-I${.CURDIR} -I${.CURDIR}/arch/${MACHINE_ARCH}
 
 CFLAGS+=	-Wall
 
-DPADD=		${LDR_INTERP} ${LIBOFW} ${LIBSAFDT} ${LIBSA}
-LDADD=		${LDR_INTERP} ${LIBOFW} ${LIBSAFDT} ${LIBSA}
+DPADD=		${LDR_INTERP} ${LIBOFW} ${LIBSAFDT} ${LIBSA} ${LIBKBOOT}
+LDADD=		${LDR_INTERP} ${LIBOFW} ${LIBSAFDT} ${LIBSA} ${LIBKBOOT}
 
 .include <bsd.prog.mk>

stand/kboot/libkboot/Makefile

@@ -0,0 +1,10 @@
+.include <bsd.init.mk>
+
+LIB=	kboot
+WARNS?= 4
+
+CFLAGS+=-I${.CURDIR} -I${.CURDIR}/arch/${MACHINE_ARCH}
+
+SRCS=	crt1.c
+
+.include <bsd.lib.mk>