pb-udhcpc: Fix generation of TFTP URLs

DHCP boot-file parameters may not contain a preceeding slash. In this
case, the current udhcp script will not form a correct URL.

This change adds the slash unconditionally; in the case that the
pathname already has a preceeding slash, the URL parser will handle this
correctly.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Geoff Levand <geoff@infradead.org>