我编译uboot时出现以下error，望高手解救
make[1]: Entering directory `/home/roots/share/u-boot-1.1.7/post/cpu'
make[1]: Nothing to be done for `_depend'.
make[1]: Leaving directory `/home/roots/share/u-boot-1.1.7/post/cpu'
make -C tools all
make[1]: Entering directory `/home/roots/share/u-boot-1.1.7/tools'
gcc -Wall -pedantic -idirafter /home/roots/share/u-boot-1.1.7/include -idirafter /home/roots/share/u-boot-1.1.7/include2 -idirafter /home/roots/share/u-boot-1.1.7/include -DTEXT_BASE=0x33F80000 -DUSE_HOSTCC -O -c -o img2srec.o img2srec.c
gcc: error trying to exec 'as': execvp: No such file or directory
make[1]: *** [img2srec.o] Error 1
make[1]: Leaving directory `/home/roots/share/u-boot-1.1.7/tools'
make: *** [tools] Error 2
