split nand.c into nand/{core,fileio,tcl}.c

Moves commands into nand/tcl.c and core implementation to 'nand/core.c'
and 'nand/fileio.c'.  Eliminates 'flash/nand.c'.

Adds 'nand/imp.h' to share routines between TCL commands and core.

src/flash/Makefile.am

@@ -12,7 +12,6 @@ libflash_la_SOURCES = \
 	arm_nandio.c \
 	nand_ecc.c \
 	nand_ecc_kw.c \
-	nand.c \
 	mflash.c
 
 libflash_la_LIBADD = \