at91dataflashboot: convert to GENTARGETS

This commit converts the at91dataflashboot bootloader build process to
GENTARGETS. It also does the following changes :

 * As the package directory name is at91dataflashboot, the
   configuration options and make variables are renamed to
   AT91DATAFLASHBOOT instead of DATAFLASHBOOT.

 * Since the board selection in Kconfig wasn't used anywhere, get rid
   of it. at91dataflashboot is only for AT91RM9200 apparently anyway.

[Peter: add ':=' -> '=' change]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>