kernel, ramdisk: some boot unification

To use the new SD building script, Linux has to be configured with
loop.max_part=15 on the command line (or set at module load time)
to make the loopback device see the partitions.

This commit removes a lot of differences between the ARM and x86
boot ramdisk and rc scripts. It changes the ARM build from running
from ramdisk to requiring a full filesystem on the SD image and
booting into it.

	. ramdisk: remove some arm-only utilities only used for running
	  from the shell
	. remove ARM-only rc.arm, proto.arm.small, ttys and mylogin.sh
	  boot-time ramdisk files
	. change kernel to add "arch" variable so userland knows what
	  we're running on from sysenv
	. make ARM use the regular ramdisk rc file, changed to distinguish
	  i386-only and ARM-only drivers; requires rootdevname to be set
	. change /etc/rc and /usr/etc/rc to start i386-only drivers only on
	  i386 systems
	. change the kernel/arm to have a special case for the memory
	  driver to load it higher so it can be bigger
	. add uEnv.txt, cmdline.txt and a for now highly linux-dependent
	  SD preparation script arm_sdimage.sh to the git repository in
	  releasetools/

Change-Id: I68910ba4e96ee80f7a12b65e48b5d39b43ca6397

include/minix/const.h

@@ -145,6 +145,7 @@
 #define INVAL_GID	       ((gid_t) -1)	/* invalid gid value */
 
 #define SERVARNAME		"cttyline"
+#define ARCHVARNAME		"arch"
 #define SERBAUDVARNAME		"cttybaud"
 
 /* Bits for s_flags in the privilege structure. */