bootutil: Fix signed/unsigned comparison in boot_read_enc_key #1929

Job	Run time
config (RELEASE 2,4,6 SIGNATURE)	9m 29s
config (RELEASE 2,4,6 SIGNATURE LOW)	14m 1s
config (RELEASE 2,4,6 SIGNATURE MEDIUM)	17m 42s
config (RELEASE 8,10 SIGNATURE)	6m 50s
config (RELEASE 8,10 SIGNATURE LOW)	9m 49s
config (RELEASE 8,10 SIGNATURE MEDIUM)	12m 25s
config (MINSIZEREL 2,4,6 SIGNATURE)	15m 35s
config (MINSIZEREL 2,4,6 SIGNATURE LOW)	19m 57s
config (MINSIZEREL 2,4,6 SIGNATURE MEDIUM)	24m 16s
config (MINSIZEREL 8,10 SIGNATURE)	11m 3s
config (MINSIZEREL 8,10 SIGNATURE LOW)	14m 12s
config (MINSIZEREL 8,10 SIGNATURE MEDIUM)	16m 43s
	2h 52m 2s

Provide feedback