Aggregate PBR related definitions and redefine as "boot_sector" to comply
with the exFAT specification.
And, rename variable names including 'pbr'.
Signed-off-by: NTetsuhiro Kohada <kohada.t2@gmail.com>
Reviewed-by: NSungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: NNamjae Jeon <namjae.jeon@samsung.com>