kbuild change breaks HiFive-U boot here

[Paul Walmsley]

Commit 858805b336be1cabb3d9033adaa3676574d12e37 ("kbuild: add $(BASH) to 
run scripts with bash-extension") breaks my kernel test flow that targets 
the HiFive Unleashed board.  The boot traps during BBL early boot and 
stops.  QEMU is unaffected.  Reverting 858805b336be fixes the issue.

I haven't yet had the opportunity to root-cause the issue.  The issue may 
be related to idiosyncracies in my local boot testing process, rather 
than this commit.


- Paul