buildman: Remove the directory prefix from each error line

The full path is long and also includes buildman private directories.
Clean this up, so that only a relative U-Boot path is shown.

This will change warnings like these:

/home/sjg/c/src/third_party/u-boot/buildman5/.bm-work/00/arch/sandbox/cpu/cpu.c: In function 'timer_get_us':
/home/sjg/c/src/third_party/u-boot/buildman5/.bm-work/00/arch/sandbox/cpu/cpu.c:40:9: warning: unused variable 'i' [-Wunused-variable]

/home/sjg/c/src/third_party/u-boot/files/arch/sandbox/cpu/cpu.c: In function 'timer_get_us':
/home/sjg/c/src/third_party/u-boot/files/arch/sandbox/cpu/cpu.c:40:9: warning: unused variable 'i' [-Wunused-variable]

to:

arch/sandbox/cpu/cpu.c: In function 'timer_get_us':
arch/sandbox/cpu/cpu.c:40:9: warning: unused variable 'i' [-Wunused-variable]

Signed-off-by: Simon Glass <sjg@chromium.org>