Fix bug that breaks booting in U-Boot v2019.10

Rename $part back to $distro_bootpart, which fixes
a bug that causes the kernel not to boot under
v2019.10.

BUG: 142311474
Test: local build and boot
Change-Id: I6abb8a5f56df5daaa7c506aed5f1a1fa0a23e93d
diff --git a/tools/create_base_image_arm.sh b/tools/create_base_image_arm.sh
index e6d0b85..b0671f2 100755
--- a/tools/create_base_image_arm.sh
+++ b/tools/create_base_image_arm.sh
@@ -139,9 +139,9 @@
 bootcmd_mmc0=devnum=0; run mmc_boot
 bootcmd_mmc1=devnum=1; run mmc_boot
 mmc_boot=if mmc dev ${devnum}; then ; run scan_for_boot_part; fi
-scan_for_boot_part=part list mmc ${devnum} -bootable devplist; env exists devplist || setenv devplist 1; if test $devnum = 1; then script_type=init; else script_type=boot; fi; for part in ${devplist}; do if fstype mmc ${devnum}:${part} bootfstype; then run find_script; fi; done; setenv devplist; setenv script_type;
-find_script=if test -e mmc ${devnum}:${part} /boot/$script_type.scr; then echo Found U-Boot script /boot/$script_type.scr; run run_scr; fi
-run_scr=load mmc ${devnum}:${part} ${scriptaddr} /boot/$script_type.scr; source ${scriptaddr}
+scan_for_boot_part=part list mmc ${devnum} -bootable devplist; env exists devplist || setenv devplist 1; if test $devnum = 1; then script_type=init; else script_type=boot; fi; for distro_bootpart in ${devplist}; do if fstype mmc ${devnum}:${distro_bootpart} bootfstype; then run find_script; fi; done; setenv devplist; setenv script_type;
+find_script=if test -e mmc ${devnum}:${distro_bootpart} /boot/$script_type.scr; then echo Found U-Boot script /boot/$script_type.scr; run run_scr; fi
+run_scr=load mmc ${devnum}:${distro_bootpart} ${scriptaddr} /boot/$script_type.scr; source ${scriptaddr}
 EOF
 	${ANDROID_HOST_OUT}/bin/mkenvimage -s 32768 -o ${bootenv} - < ${tmpfile}
 fi