binman: Correct handling of zero bss size

Fix the check for the __bss_size symbol, since it may be 0. Unfortunately
there was no test coverage for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/Makefile b/tools/binman/test/Makefile
index cd66a30..4d152ee 100644
--- a/tools/binman/test/Makefile
+++ b/tools/binman/test/Makefile
@@ -32,7 +32,7 @@
 LDS_EFL_SECTIONS := -T $(SRC)elf_sections.lds
 LDS_BLOB := -T $(SRC)blob_syms.lds
 
-TARGETS = u_boot_ucode_ptr u_boot_no_ucode_ptr bss_data \
+TARGETS = u_boot_ucode_ptr u_boot_no_ucode_ptr bss_data bss_data_zero \
 	u_boot_binman_syms u_boot_binman_syms.bin u_boot_binman_syms_bad \
 	u_boot_binman_syms_size u_boot_binman_syms_x86 embed_data \
 	u_boot_binman_embed u_boot_binman_embed_sm elf_sections blob_syms.bin
@@ -48,6 +48,9 @@
 bss_data: CFLAGS += $(SRC)bss_data.lds
 bss_data: bss_data.c
 
+bss_data_zero: CFLAGS += $(SRC)bss_data_zero.lds
+bss_data_zero: bss_data_zero.c
+
 embed_data: CFLAGS += $(SRC)embed_data.lds
 embed_data: embed_data.c