rom.sh: rename mkvendorfiles

it mainly does general tasks, like handling utils
and enabling ccache. the vfiles are a small part.

rename the function accordingly. it is called by
premake, so let's call it corebootpremake.

this change will also make sense when cherry-picked
into cbmk, which does not handle vfiles at all.

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/data/coreboot/mkhelper.cfg

@@ -11,6 +11,6 @@ v="$v displaymode tmprom newrom payload_uboot"
 eval `setvars "n" $pv`
 eval `setvars "" $v`
 
-premake="mkvendorfiles"
+premake="corebootpremake"
 mkhelper="mkcorebootbin"
 postmake="mkcoreboottar"