First, you need recent u-boot (git) development tree. How to get it:
http://wiki.davincidsp.com/index.php?title=Linux_Toolchain#Get_recent_open_source_U-Boot
(the page is for DaVinci, but it mainly applies to BeagleBoard as well)
Then apply the BeagleBoard specific patches (which are not yet
integrated in above u-boot development tree):
http://elinux.org/BeagleBoard#Patches (U-Boot section)
Once you downloaded patch1.diff.txt to patch4.diff.txt, apply (patch)
above u-boot development tree with
patch -p1 < patch1.diff.txt
patch -p1 < patch2.diff.txt
patch -p1 < patch3.diff.txt
patch -p1 < patch4.diff.txt
See e.g.
http://wiki.davincidsp.com/index.php?title=Working_with_Linux_patches#Applying_a_patch
for more patch details.
Then, you should be able to build patched u-boot for BeagleBoard with
make distclean
make omap3530beagle_config
make
Best regards
Dirk