dtimg/am57xx_evm_defconfig: Rename dtimg to adtimg

Rename the existing 'dtimg' command to 'adtimg', in order to:
 - Suggest the Android origins and scope
 - Be consistent with the upcoming 'abootimg' command (naming
   suggested by Simon [*])

The change in _not_ backward compatible, but its benefits outweigh its
downsides, given that we don't expect active users of 'dtimg' today.

Perform the rename in several steps:
 1. Rename *.c file and Kconfig symbol. This should allow
    'git log --follow' to properly track the history of 'adtimg.c'
 2. 's/dtimg/adtimg/g' in the internal namespace of 'adtimg.c'

ELF comparison [**] before and after shows no functional change.

[*] https://patchwork.ozlabs.org/patch/1182212/#2291600
[**] diff -u <(objdump -d cmd/dtimg.o) <(objdump -d cmd/adtimg.o)

Cc: Tom Rini <trini@konsulko.com>
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Simon Glass<sjg@chromium.org>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>

common/Makefile

@@ -117,7 +117,7 @@ obj-$(CONFIG_IO_TRACE) += iotrace.o
 obj-y += memsize.o
 obj-y += stdio.o
 
-obj-$(CONFIG_CMD_DTIMG) += image-android-dt.o
+obj-$(CONFIG_CMD_ADTIMG) += image-android-dt.o
 
 ifdef CONFIG_CMD_EEPROM_LAYOUT
 obj-y += eeprom/eeprom_field.o eeprom/eeprom_layout.o