boot_emmc_devkit: Use the correct uboot

We want to use the devkit's uboot, not the phone one.
5 jobs for uuu in 5 minutes and 2 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #267609
librem5
build:deb

00:01:29

 
  Test
passed #267649
librem5
test:download:deb

00:01:32

passed #267650
librem5
test:download:pip

00:02:00

failed #267610
librem5
test:download:deb

00:00:32

failed #267611
librem5
test:download:pip

00:00:59