Hi,
I followed these instructions to build an U-Boot for my beaglebone:
http://www.eewiki.net/display/linuxonarm/BeagleBone
http://dev.gentoo.org/~armin76/arm/beaglebone/install.xml
Both do not mention the kind of shell they use to build the U-Boot.
Normally I am using zsh instead bash of tcsh. When using zsh, the
build fails leavong an U-Boot behind but no MLO.
Futhermore the cross compiler suit used for this task often is
abbreviated with sometying like
arm-linux-gun
which may or may not be installed on the system.
With gentoo and gentoos crossdev I built a cross toolchain
which compiler is named
armv7a-hardfloat-linux-gnueabi-gcc
. I also have a toolchain installed called, ahich compiler is called
armv7a-softfp-linux-gnueabi-gcc
Only when switching to
bash
and using armv7a-hardfloat-linux-gnueabi- as the toolchain prefix the
compilation ended successfully and the U-Boot and MLO were built.
I would suggest to either add the shell dependency to the tutorials
above or -- better -- remove this dependency from the U-Boot makefile
system.
Only my two cents...your linux board may vary... 
Best regards,
mcc