omap3_beagle: replace uImage.beagle with generic uImage

e6829308
(BeagleBoard: config: use uImage.beagle for tftp)

Introduced uImage.beagle which does not happen to be default output
file of Linux kernel build make uImage (output is uImage).

So, replace uImage.beagle with uImage

Signed-off-by: Nishanth Menon <nm@ti.com>