km/scripts: search for kernel/DTBs at serverip:/PRODUCTNAME via TFTP in develop mode

Search for the kernel and DTBs in a folder named PRODUCTNAME (found in the IVM)
at the TFTP server instead of the u-boot boardname.

Signed-off-by: Tobias Müller <Tobias.Mueller@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Reviewed-by: Heiko Schocher <hs@denx.de>