packages: uboot-mxs: override instead of appending u-boot make flags

packages: uboot-mxs: override instead of appending u-boot make flags

This prevents passing down the HOSTCC stuff set in u-boot.mk
which results in linking errors against openssl:

tools/mxsimage.o: In function `sb_aes_reinit':
mxsimage.c:(.text+0x202): undefined reference to `EVP_CIPHER_CTX_reset'
tools/mxsimage.o: In function `mxsimage_generate':
mxsimage.c:(.text+0x110d): undefined reference to `EVP_MD_CTX_new'
mxsimage.c:(.text+0x114f): undefined reference to `EVP_MD_CTX_free'
mxsimage.c:(.text+0x11c3): undefined reference to `EVP_MD_CTX_new'
mxsimage.c:(.text+0x1323): undefined reference to `EVP_MD_CTX_free'
mxsimage.c:(.text+0x134a): undefined reference to `EVP_CIPHER_CTX_reset'
tools/mxsimage.o: In function `mxsimage_verify_print_header':
mxsimage.c:(.text+0x23ce): undefined reference to `EVP_MD_CTX_new'
mxsimage.c:(.text+0x242c): undefined reference to `EVP_MD_CTX_new'
mxsimage.c:(.text+0x246b): undefined reference to `EVP_MD_CTX_free'
mxsimage.c:(.text+0x24ef): undefined reference to `EVP_CIPHER_CTX_reset'
mxsimage.c:(.text+0x2e52): undefined reference to `EVP_MD_CTX_free'
collect2: error: ld returned 1 exit status

Signed-off-by: Michael Heimpold <mhei@heimpold.de>