manifest: Point to tip of master after snapshot

PR https://github.com/zephyrproject-rtos/mcuboot/pull/8

 was mistakenly
merged instead of pushed, which made the last 4 commits of zephyr's
mcuboot divergen in SHA with respect to the upstream ones.
In order to preserve the status of the zephyr mcuboot repo as an
exact mirror of the upstream one, a snapshot has been taken to preserve
the current SHA (0f4463caec3c3a8fac1350d0d6dfbee678a8ce06) and then the
master branch has been reset to the upstream equivalent
(7e7b4ad1999bf148f88059195f8d2e1d3dccf190). This commit switches the
pointer to the latter, but the mcuboot code itself is identical.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>