ash: add bash-compatible EPOCH variables

ash: add bash-compatible EPOCH variables

Bash 5.0 added the dynamic variable EPOCHSECONDS and EPOCHREALTIME
which return the number of seconds since the Unix Epoch as an
integer or float.  These are useful for logging or tracing.

function                                             old     new   delta
change_epoch                                           -      78     +78
.rodata                                           175167  175235     +68
varinit_data                                         264     312     +48
change_seconds                                         -      24     +24
change_realtime                                        -      24     +24
------------------------------------------------------------------------------
(add/remove: 3/0 grow/shrink: 2/0 up/down: 242/0)             Total: 242 bytes
   text	   data	    bss	    dec	    hex	filename
 938508	   4203	   1888	 944599	  e69d7	busybox_old
 938702	   4203	   1888	 944793	  e6a99	busybox_unstripped

v2: Cast tv_sec and tv_usec to unsigned quantities.
    Add brackets to macros.

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>