dtimg/am57xx_evm_defconfig: Rename dtimg to adtimg

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>