pxe: fix handling of absolute paths

pxelinux and syslinux differ in their handling of absolute paths in menu
files. A pxelinux path is aways prepended with the bootfile path while
syslinux allows for absolute paths. u-boot was always treating a leading
/ as an absolute path breaking some pxelinux setups. Fix this by adding
a flag to distinguish pxelinux vs. syslinux behavior.

Reported-by: Ian Campbell <Ian.Campbell@citrix.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>