uboot-omap: Do not hide bootloader packages

Because they were hidden, there was no way to disable the uboot
targets in menuconfig, so they had to be built every time. The omap
target is the only one to hide uboot packages. To be consistent with
the other targets, and have more control over the build, un-hide the
u-boot packages.

Note that the default behavior remains unchanged, as uboot will be
built unless explicitly disabled.

Signed-off-by: Alexandru Gagniuc <alex.g@adaptrum.com>