warp7: include: configs: Specify image name of bootscript in FIT

warp7: include: configs: Specify image name of bootscript in FIT

When obtaining the bootscript from a FIT image we need to specify the name
of the bootscript as defined inside of the FIT.

This patch makes a define that appends a "bootscr" parameter to the source
command when compiling up in FIT mode on warp7.

An environment variable is supplied to enable others to use a different
name than "bootscr" as the image name of the boot script in their FIT.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>