MA-15860 Get and switch the target dev in fastboot erase

MA-15860 Get and switch the target dev in fastboot erase

Target mmc dev is not properly switched when the device enters
fastboot mode via uuu "-i" parameter, which causes "erase"
operation doesn't work.

Get and switch the target mmc dev every time before erase operations
happen.

Test: "fastboot erase boot_a" in uuu fastboot mode.

Change-Id: I4822d2b4ecfd2d874dfbe7474d6824b8fc3a7903
Signed-off-by: Ji Luo <ji.luo@nxp.com>
(cherry picked from commit 46161b62c341139dcdbb13e00634ae8ae903c5da)
(cherry picked from commit b66d13c7ecd1c1e44dd9edb6c0d5566b4f738bbe)