clang detects that system char signedness will affect program runtime:
scripts/kwboot.c:395:10: warning: result of comparison of constant 255
with expression of type 'char' is always true
[-Wtautological-constant-out-of-range-compare]
                if (*p != 0xff)
                    ~~ ^  ~~~~

Fix this by using uint8_t where appropriate.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>