include: configs: Override get_fit_config to get FIT config for AM57x

include: configs: Override get_fit_config to get FIT config for AM57x

Kernel commit 837833a724b7 ("environment: ti: Add get_fit_config command
to get FIT config string") introduced "get_fit_config" in ti_armv7_common.h
to mangle the fdtfile name when used to select a config node from the OE
made FIT image. However, the ti_armv7_common.h is common for both K3 and
AM57xx platforms. AM57xx platforms' fdtfile name does not have '/' and
"conf-" prefix so the setexpr command fails and boot hangs.

Override the get_fit_config in AM57x specific config header to get the
correct FIT config name.

Signed-off-by: Sinthu Raja <sinthu.raja@ti.com>
Signed-off-by: Anurag Dutta <a-dutta@ti.com>